Yes, the broader context of that email chain is a broad unloading of a 'greatest hits' of a decade of grievances against SparkFun (the meme site mentioned was made in 2017, by an employee who no longer works there).
I quoted the lines that seemed to most obviously tip that particular email exchange beyond a measured harassment report (as originally implied), crossing the line into what could be reasonably considered 'unappropriately aggressive behavior' (to quote the SparkFun CoC).
I would agree that the ex-employee's 'Sincerely, Fuck Off' was similarly aggressive, but less relevant since he's no longer an employee anyway, and it seemed pretty clear that he was the one being subject to targeted harassment in this instance (having accusations being forwarded to his partner's employer) rather than the other way around.
Not to split hairs here but it doesn’t look like he said it was a measured harassment report. Having worked customer service and HR, it’s somewhat common for people reporting harassment to be irate or even downright rude, especially when they feel that they’re being ignored.
The idea that calling someone a jerk is grounds for a company to ignore a serious complaint is, paradoxically, what some people describe as their reason for being rude.
Anyway I don’t know what’s going on here but “Sparkfun has ceased its business relationship with Adafruit because a guy called us jerks in a complaint about our CEO” would be hilarious