SFP plays against a Bowser that couldn't figure out how to escape ZSS's "standing chaingrab". PM me if you want to know more about that.Just curious: What Bowsers have you played?
In my experience, a 70-30 MU is one where a moderately good player can defeat a great player who uses a terribly disadvantaged player. I'm gladly the first to admit I've only played moderately good to good Zamuses, and I haven't played the competent Zamus mains in Texas yet (which is kinda funny since we have so many now, haha). But that said, I have yet to feel Zamus shuts down Bowser in the same fashion that Diddy Kong does, for example, and absolutely not more than ICs.
I forget off the top of my head which Zamus main KingKong MM'd at Apex, but he had also been believing the MU to be 50-50, and he reinforced his notion at Apex. I'll have to look it up and get back to you.
KingKong played Snakeee at APEX (namesearch flooding yeah).
I asked SFP how it was possible for KK not to lose that and he said Snakeee didn't know the matchup. I have serious doubts SFP knows the matchup any better than he does, because the way he puts its, all the ZSS has to do is play normally and she'll shut out Bowser. He also got a lot of key info on how Bowser plays the MU wrong, so...
I don't know if you've seen it, but NRiddle posted a vid on how to DSmash gimp the whole cast including Bowser. He apparently didn't see anything wrong with how Bowser was recovering while editing the video. It's gotten to the point where I take anything ZSS mains say about low tier characters with a grain of salt.
edit: BTW; I think Bowser is disadvantaged in the MU. But I'm not sure on exactly how much.