Is the 280T variant the HGW variant?
Probably not in the terms you consider HGW.
A350 was built in batches. Current batch is using all improvements gathered so far, including 280T. Any A359 delivered nowadays is capable of 280T.
Previous batches were built with different MTOW. First two (around 20 planes) I think were 268T birds, and then they introduced 275T. These numbers are true to any bird within a batch.
There was also one special batch for Philippines, I think, for a connection to New York, upgraded to 278t, if I remember correctly, but a 275t batch bird. It never found another customer, as 280t bird was already in research.
However, A350 allows for a paper change of MTOW - that is, you could buy the very same frame with reduced MTOW (and ability to reach max allowed MTOW). It depends on mission profile, routes flown, generally airline profile for A359. There's a number of WV (Weight Variants) allowed for any A350, including A359. I guess around 20 now. If you don't need 280t, you can select (and pay for) lower weight variant.
So when talking about specific fleet, the important things are build batch, and weight option selected by airline. There's no such thing as HGW (Boeing style), though.
Hope that clears.