This is a great compressor, probably my favorite VST. The sidechain high pass filter knob allows preventing bass frequencies from triggering the compressor, for most presets it is set to around 200Hz... this made me wonder: do other compressors without this knob, such as the regular OldTimer, have a hard coded high pass filter built-in?

I don't know the answer but you should be easily able to test yourself if you want:

Feed it a sinewave and sweep the frequency around and see where it's triggered (or not).

True, I'm more curious to see if it is a common design feature in all compressors (including hardware ones). It wouldn't really make sense because then they wouldn't be very effective on bass. So if this is something unique to the oldTimer ME, then I am using it in a fundamentally differently manner than other compressors and am wondering how much this accounts for its 'sound'.

Adjustable sidechain hipass is certainly not so uncommon in compressors, for example I use ReaComp often and it has it too.

About hard-coded (invisible) SC filters and Hardware I don't know, but me at least I wouldn't like that probably because as you said it might make it unsuseable for bass compression (depending on where the cutoff frequency would sit).

To answer the OP's question -

Yes, compressors typically weight or favor certain frequencies more than others. For example the SSL bus compressor really responds to low frequencies, you have to be careful. In this way, it can help guide your mix because if there is too much bass in your mix, the compressor will pump the hell out of the sound.