According to "European Organisation for Safety of Air Navigation" and FAA, on TCAS all Resolution Advisories are inhibited below 1000ft.

https://skybrary.aero/articles/tcas-ii-ra-very-low-altitude

https://www.faa.gov/documentlibrary/media/advisory_circular/...

Comments on Reddit from Boeing pilots suggested that TCAS wouldn't sound on the ground (or that near to the ground).

Me too. Usually the pilots will mention it if they receive an RA, though. I think TCAS might even be disabled (or nerfed?) at low altitudes like this -- not positive, though.

From what I've read, it sounds like this airport is not equipped with the ATC-equivalent of TCAS -- which I think would have been sounding all kinds of alarms (how early?) in the tower for a scenario like this.

TCAS is muted/disabled on the ground and at this altitude.