I think I'd want an official statement, not an anonymous statement from an unknown source.
The FSB had had the team following him for quite some time. We know from phone call records that those people were communicating with people pretty high up.
If it isn't reaching Putin, then it's some very high-ranking people in the his administration independently deciding to knock off his political opponents.
It's not impossible that they'd do that, but given the domestic political ramifications, I have a hard time believing that they'd want to just do it without checking with Putin, because it could clobber him politically as well.
EDIT: Oh, they're saying that him dying in prison shortly after being imprisoned rather than his earlier attempted murder by the FSB might not have been directed by Putin. I guess that that's at least more-plausible. Doesn't mean that he wasn't murdered, but I could believe that Putin would have been fine with him just being permanently imprisoned.