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