I’ve gotten several comments deriding me referring to an Elizabeth Line station as “a tube station” but so far—despite my requests—nobody’s explained WHY.

Is it NOT considered part of the Underground?
It has the same logo but in purple, which I assumed was just for Queen reasons.

It interfaces WITH Underground stations, does it not??

Is this a colloquialism I just won’t understand?

@Graham_LRR The Elizabeth Line/Crossrail is a lot closer to the London Overground than the Underground/Tube. It’s actually part of the National Rail system (which is why your Eurail pass worked on it), not the Tube.
There were many people annoyed when the name was announced as the Elizabeth Line because it would lead to this confusion ([blank] line was previously reserved for tube lines)