Is there a reason we can't see ml c/technology from hexbear?

I don't think there is. From what I've been able to tell, it's a bug and is relatively common. There are certain comms and even certain users that are plain invisible, as it seems as if they do not federate correctly (despite the rest of the instance being federated). I have talked about it before, and c/technology isn't the only one like that. We also can't see the lemmy.ml privacy comm from hexbear. And they can't properly see our literature comm. I'm sure there are others, in fact I know there are I just can't remember which atm.

Are we able to defederate from specific communities?

I think we actually are able to do that now? But that's not what this is. This was an issue long before that was a feature. And like I said, it happens with certain users too and there is nothing in the modlog of either instance about them being banned or something. They simply don't exist if you're looking for them from the other instance.