Now how would a 90ies XC bike compare with a modern bike geometry and its components, not very well. 😁 Have a lefty carbon that's almost as good but its 7 years old and not as versatile as the gravel bike.
My orange p7 from early 2000s (26 inch wheels) has the exact same geometry as my gravel bike other than the stem/bars. I've put them next to each other, frame and angles line up almost exactly. It's a marketing thing.
Gravel is a poorly defined category at the best of times, and hardly exists in the UK for most people. What we do have is singletrack, bridleways, towpaths, disused railway lines (the closest to USA gravel) and a hell of a lot of road connecting it.
And for half of the year most of that turns to mud and requires different tyres at least.
An xc hardtail mtb is a far better fit for most offroad riding in the UK than most gravel bikes imo, provided you're not going to ride 10+ miles on road to get to it first.
MTBs can take gravel style fast rolling 38-40c tyres that work OK on-road too and make a lot of difference in summer at least if you're doing road segments.
What really kills versatility in all aspects imo is the mania for 1x drivetrains which make a lot of gravel bikes sold unsuitable for dual duty as road bikes for group rides. Same for mtbs all going 1x really, I never had a problem with 2x or even a triple.