as far as i'm aware it is not, because:
- they use a variant of lemmy which literally doesn't federate with the rest of lemmy (because it was developed and forked before that was even a thing--they're a very old instance), and they don't have the developers to rectify that;
- and separately, there's not... really? a desire on the part of anybody to federate, either on hexbear's side or on lemmy's. it'd be far more beneficial to lemmy than hexbear, anyways