It's been established that he was indeed poisoned. Numerous reputable institutions from several countries have confirmed as much.
Late last year, Bellingcat unearthed somebody associated with the FSB whom Nawalny – posing as a ranking officer – then called, tricking the man into implicating himself in an attempt to destroy evidence for a poisoning. You can listen to the call on Youtube. Apparently, the FSB guy was on sick leave with Covid-19; maybe that's why he let his guard down.
There seems to be a lot of evidence pointing towards the involvement of the FSB. Nawalny says only Putin could've ordered an attempt on his life. Take it for what you will. Personally, I'd also find exploring the possibility of a rogue FSB unit worthwhile.