Why is more isolation negative, should be a spectrum, 98% alone and the rest socialising.

The abstract doesn't say that isolation is negative (I think). It just says WFH folks are more socially isolated.

For some people, more social isolation is OK. For others, not so OK. YMMV .

I personally think that more socializing is better, if it's with people who I become better by being around. The tough part is knowing who's good for me, and how I can find them.

I prefer being alone but also feel worse after prolonged periods alone.

Preferring something doesn't mean it's good for you.

Well, the abstract also says they were not feeling well because of that, I guess that's what matters