Is calling someone a "stinky manager" a personal attack?
yes!
yes!
"software developers are stinky managers" is not a personal attack. neither is "managers are stinky", but "you/he/she/etc are something" is personal. it may not be an attack though if it is still part of a generalization: "you are a stinky manager because all software developers are stinky managers". but the article is not making that generalization, and therefore it's not just personal but also an attack.
What Kelley would have said if he were acting like an adult & offering professional criticism was that Jarred was a bad manager. Ineffective, even. That is what an adult would have said there.