alt Hacker NewsOPs issue was more clearly directed at "A week or two ago I was surprised to see a Google Gemini summary at the top of my email on my phone", which Mimestream doesn't include.
The rest of the complaint was about a bug that happened, just today, with sorting and spam control.
OP meaning the author of the linked article? Maybe I missed the mark but I felt his main rejection of Gmail ended up being because they train their AI based on his private and or commercial data.