No, you probably haven't read the conversation piece. The post is ultimately about switching providers because Google's service crosses a line from (1) targeted advertising to (2) using personal and confidential information for model training.
A service to clean up the UI does nothing to solve the issue at hand.
The dealbreaker was data usage for AI training, not UI:
"We are going to use your email to train our LLMs. I'm not okay with that... my confidential commercial information is NOT okay to use to train your models [...] So... goodbye Gmail."
OPs 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.