He's got a fix for it, see, but those big meanies at Google won't let him implement it. He's even encouraging you to yell at Google on his behalf. (It turns out, Google is a bit shy about letting third parties have access to fiddle with your Gmail inbox settings. Can't say I blame them for that.)
You could sign the petition. Or you could not bother. Me, I'm not going to bother. Google is probably deaf to policy complaints like this -- or at the very least, Seth has a tall mountain to climb. Instead, consider that Google already has two ways that you can address issues with promotional tab placement -- and neither of them involve a third-party petition.
- Take that individual message that you think belongs in "primary" and move it there. If enough people do that, Google will re-think where it places same and similar messages in the future. This process already exists to provide feedback to Google, direct input into the algorithm.
- Disable tabs, if you don't like the tab decisions Google is making with regard to your Gmail inbox. (Options: Configure Inbox: Turn off all checkboxes except Primary.) Gmail works just fine without 'em, I know from personal experience.
Not to mention -- usage of the promotions tab by Gmail users is not universal. In 2017, a Return Path survey suggested only a third of Gmail users used tabs, and industry data and discussion since then suggests that this number continues to drop.
For more thoughts on how to deal with life in the Gmail Promotions tab (or how to escape), most of what I was able to round up in 2018 is still applicable.