A slew rate calculator says that for an arbitrary 100MHz signal a minimum slew rate of 1884V/μS is required.
This result isn't for an arbitrary 100 MHz signal. It's for a 100 MHz signal with a specific amplitude.
If I used an amplifier with that exact slew rate is it also a low pass filter?
It won't be very effective.
For one, if you input a 200 MHz signal with a amplitude less than 1/2 the amplitude you used in the calculator to get your slew rate limit, it won't be affected by the slew rate effect and won't be filtered. So for low amplitude signals, this filter won't have the same cut-off as for larger signals.
More generally, the slew-rate limiting effect is a nonlinear effect. If you input a 125 MHz signal at a high enough amplitude to cause slew-rate limiting, it wouldn't just attenuate the 125-MHz component, it would also add harmonic frequency components at higher frequencies, so if your goal is not to have high frequency content in the output, this filter would have made things worse rather than better.