It also says there is 145 people online - Does that mean roughly 2/3 of active users vote stuff to hot? ~100 people holding up a niche community with a fraction of those the posters themselves.

For this, it's important to remember that's the number of people online at that very moment, but the vote count is persistent. Any number of those upvotes could have come from users who aren't online presently, but had been online an hour or two prior. ~66%, in this case, is not the actual amount, it's just the upper bound.

I always baselessly suspected that Reddit fluffs up the numbers to make engagement seem like it is much greater than it is, but this is 1000x smaller than the sub count suggests.

It's possible, and very plausible, that they do this, but it's much less plausible (though still possible) that they do it to that degree.