Technically, yes, it is true... with caveats.
A TS will, in fact, short across the Right, Mic, and Ground connections. However, it is extremely unlikely (although possible if they made them in an absolutely idiotic way), that it would severely damage your phone. The reason it is doubtful is that it is making a connection to the ground, not just the mic, which means that the signal is not feeding from the right channel to the mic input, but, rather, both are shunted directly to ground. However, because of that shorting, your cable may actually work.
That said, it won't work very well. If i understand correctly, you are wanting to feed the signal into an audio mixing console, or something similar. The problem is that the phone provides a headphone-level signal (amplified), and your console is expecting a mic or line level signal (essentially unamplified, possibly buffered) through that XLR input. If you turn the volume way down on your phone, and your console is decent, it will likely work, in that aspect. If your cable length is very long, though, you'll be picking up quite a bit of noise in the wire, as your design defeats the noise-cancelling balancing typically utilized in that scenario.
To make the proper cable, you would be better off using an actual 1/8" TRRS connector (readily available), and wire the Right and Left signals (as well as the ground, obviously) to 2 1/4" TS plugs. Most audio mixers will handle a much hotter signal on their 1/4" inputs. A Direct Box could even be used to take advantage of the balancing available on a XLR cable, and, since most Direct Boxes have a pad switch, it may even match the level, as well as the impedance.
i should add that i don't have an iPhone 4S... just an old 3G... i have long switched to an Android phone. So, i cannot do any tests or measurements on your exact phone model. But, i, and many others, have used TRS connectors on the 3G, which shorts from mic to ground, with no problems - it is designed for that. And, it is impossible to create feedback with a TS connector, since they are all shorted to ground. i will advise, however, that you not use the TS connector for long periods, as some testing of the 3G at higher volumes indicates that the current on the shorted output might be enough to cause overheating. i recommend using a TRS connector, at minimum. The ring contact on the connector should be left unconnected to the cable.
Update, referencing the equipment being used:
The Fohhn Software does not transfer audio, nor does it directly process any audio. It is merely a control network, according to the Fohhn website and technical documents.
The signal chain in your installation is, likely, like this (simplified):
iPhone --> Allen&Heath mixer --> Fohhn Amplification System
The Fohhn Software controls the EQ settings (among other things) for the system as a whole.
You should be using a 1/4" plug, as i previously suggested, plugged into one of the 1/4" channel inputs on the mixer. Although, the XLR on that mixer should also work, as per my recommendations and caveats above. Start with the iPhone volume reduced, and bring up as necessary. If a lot of noise is produced, try unplugging the iPhone from it's charger. If that doesn't get rid of the noise, it is most likely that your cable is the culprit.
Use the EQ settings on the mixer, itself, to "tweak" the settings, if it is even necessary.
It reads like you might be trying to make EQ changes within the Fohhn software before the event, offsite (not while actually using the sound system). That won't work, because the Fohhn software doesn't actually process audio. It is possible to make an EQ scene for the Fohhn system, specifically with your sound file in mind, but i don't recommend that approach, as it is a systemic change to the entire system, not just that particular source.
If there is EQing that MUST be done beforehand, use audio editing software (like "Audacity" which is free) on the audio file itself. But, i highly doubt that would be necessary, as the channel EQ on the mixer should be far more than adequate, and, if frequently needed, the settings can be reproduced in a matter of seconds.
Your last comment/description also reads like you might be trying to play a sound file through the system, so that you can create an EQ setting that will work well with your IMAM's vocal microphone. If so, you can certainly use this to get in the ballpark, but keep in mind that your recording is likely to be compressed, and of limited frequency bandwidth. i would still advise using the mixer's channel EQ if there are other sources being used, other than the one vocal microphone.
With either situation, the cable should work as i described above, and a TRS connector into the iPhone is recommended. Is your question satisfactorily answered?
If you have need of more assistance on the equipment, please be sure to list the exact model numbers of the equipment you are using, not just the manufacturers.