The suspects are anarchists, they generally despise socialist governments, and most of all they despise both modern Russia and what was the USSR.
If a random Russian guy is willing to bankroll your anarchist LARP, plenty of disaffected young morons will happily take that money under the banner of anarchism.
Seems to me like you're giving too much credit to ideological inconsistency here.
I know european anarchists and they're as ideological as it gets, much more than the average communist as commies are usually more pragmatic. The disillusioned anarchists simply quit the lifestyle as there's nothing to benefit from it if you don't strongly believe in it: only hardship, unpaid work, and sacrifices.
Russia has been committing arson attacks throughout Europe, it's totally fair to suspect them and "it was actually anarchists" shouldn't obviate that suspicion nearly as much as you're suggesting it should.
https://www.nbcnews.com/news/investigations/russias-brazen-i...
https://www.theguardian.com/world/article/2024/may/30/europe...
https://sg.news.yahoo.com/3-arrested-kremlin-plot-stage-1417...
"It was committed by anarchists" just doesn't address the hypothesis at all.
Anarchists wanted the Tsar gone. Didn't work out for them in the long run.