Because 'researchers' and 'russian spy agency' sounds very different. Former are motivated by desire to shed light on questionable behavior, the latter to inflict damage on their adversary (while engaging in worse practices themselves).

> How does talking about the messenger change the message?

We need to judge the messenger's integrity in order to judge the message's accuracy. Putin might say there are no Russian soldiers in Ukraine.

We can't rely on plausibility. Good propaganda is plausible, and the Russian intelligence services are very good at propaganda.