I think these are the important paragraphs to note, before this becomes the usual “eh, should have applied in time” topic

Sellers was aware that Brexit might affect his right to stay in the country and, without any communication from government authorities, he considered his best option was to apply for Swedish citizenship.

This he did in September 2021, before the Brexit deadline of 31 December that year.

Not only was his citizenship rejected, but two subsequent attempts to remain in the country under family rights in national migration laws via his partner, now his wife, were also turned down.

So by the time he applied he lived in Sweden already 8-9 years or so. He also likely had a job I assume, would be nice to know the grounds for rejecting his application. There are of course valid reasons to not grant it, but given how terrible of an organisation Migrationsverket is (yes, repeating myself a lot in these posts, I know), I would not immediately assume that the guy did this the wrong way.