Is ethernet baseband line coded without the presence of multiple carrier frequencies signaling over the line?
The ethernet you're probably referring to (the one over twisted copper cable with RJ45 connectors) is indeed not using multiple carriers!
For instance, cable TV uses multiple carrier frequencies
each representing a different
line coded signal.
Not line coded, but passband-modulated. What happens in TV is relatively complex, and not just line-coded states. This might lead too far!
You quote wikipedia, and that's generally a good idea. But:
In digital baseband modulation (line coding) used to transmit data” and “ the voltage on the line is switched between two amplitudes (voltage levels) representing the two binary digits, 0 and 1, and the carrier (clock) frequency is combined with the data
As a communications engineer: No. That's not what "baseband modulation" is. Baseband modulation is simply the same thing you see on the carrier, but not shifted to a carrier frequency, other than 0 Hz. So, it does not extend from carrier frequency +- Bandwidth/2, but from -bandwidth/2 to +bandwidth/2. That's baseband.
Even for line coding, which the author of that paragraph confuses with baseband modulation here, the restriction to two states is too restrictive. For example, USB1/USB2 has line coding, but it has not only two states. So, maybe just ignore that paragraph.
I also assume the physical layer implementation influences the signaling method.
I'd go a step further: it fully defines it.
It does not seem like the signaling method described above can combine multiple line coded signals over the same signaling lines. Is this correct?
Ah, because the paragraph is hm, say, alternative facts, it's hard to say what it actually says about the superposition of multiple signals.
As a matter of fact, you can do multiuser modulation in baseband. It's actually what your base station's LTE chipset does internally! It generates a baseband with a signal that contains the all the signals for all the different phones. And then, it mixes that up to a carrier frequency.
But: if we're talking about systems where "line coding" is the right term, yes, these are typically not meant to "superimpose" multiple signals at the same time. However, the methods used for line coding often do go hand in hand with how multi-user access to the shared medium is still done:
For example, if you were a phone grid operator in the 1970s/1980s, you would be transforming your core network to digital signalling, which really meant sending "highs" and "lows" (as voltages or currents) to signify 0 and 1 bits. You needed to use line coding, so that clock can be recovered by the receiver, and so that you're not sending more highs than lows – because that would mean there would be an average current in one direction more than the other on the line, and that would have been bad, power-wise, and would have made decoupling systems through capacitors or transformers impossible.
Now, the job of these trunking lines was to carry much data, from many stations connected to the line! And that was perfectly possible by just assigning each station a recurring timeslot in which it was allowed to send its line-coded data (PCM telephone audio from many calls!). That needs to go hand-in-hand with the line coding: the knowing-when-to-send depends on each station having the same understanding of "when" and "how long", which is not the same if you're far enough apart that speed of light in cables matters, and the accuracy of even your best oscillators isn't good enough that over billions of bits, you don't drift apart a single bit.
So, really, line-coding is something you do when you have a physical cable-style medium that you're using in a manner that requires low-complexity reception, so you want to directly map your data bits to discrete physical states, and you need the signal on the medium to have some statistical properties.
Coming back to ethernet: Your IEEE802.3ab, also known as 1000BASE-T or just "Gigabit Ethernet". There's line coding done on it, and typically the symbols are PAM-5 (so, five different amplitudes that are combined with previous symbols to mean 4 different things, depending in which order they appear, that's the line coding at work!).
But: that line coding that makes out of 2 data bits (which can mean only 4 different states!) one of 5 different states, depending on history, that'd be called "scrambler" in e.g. a TV system, or LTE. The line gets really blurry: If you hear "line coding", you'd think the receiver just takes the state it measures, and that directly "means" some combo of bits were sent. Far from it for Gigabit Ethernet: you need an equalizer to first undo the effect of, due to propagation effects on the long cable, the symbols "seeping" into each other. Equalization is a techique that you'd know from wireless communication demodulation systems such as DVB-T, LTE…