You're mixing multiple subjects here, one being the logistics of blocking a federated system like Lemmy, the other being whether the wrong person finds the content of such a system objectionable and labels it a "national security issue."
I'm being a tad pedantic here, but my reason for pointing this out is that I think #2 is not far fetched at all, but I'm unsure of how feasible #1 might be and would love if somebody who knows more than I do would chime in.
EDIT: Looks like some have already discussed #2 in the other comment thread started by Teknikal.
Is there some way of safely circumnavigating these types of blocks in countries under oppressive regimes? I know about VPNs and TOR, but are those methods actually safe?