If we believe that 50+1 is what stopped them, because members get the final say, then how can you say they couldn't have joined even if they wanted to? If 75% of the club's voting rights belongs to the club's members, then that
is the club. If
they wanted it, they could have it.
Ultimately, I'm not even sure that's the main reason they didn't join, even if down the line that likely would have prevented them from joining. I know that sounds a bit silly in itself, because you can't really separate those two decision processes. If they knew their members wouldn't agree to it, then why pursue it? Nonetheless we know the club's management were pretty involved in the planning process of the Super League a couple of years ago, whatever shape that league had back then. So we know their management probably likes the idea of a super league in principle.
However we also sort of know they must have distanced themselves from the idea some time ago. Why else would Real Madrid and co-conspirators not include them in the planning of this final push for the ESL, even though they were heavily involved in laying the ground work just a few years prior? They must have known Bayern wasn't really on board anymore, but hoped that agreeing with 12 clubs and getting if off the ground may change their minds spontaneously. That article
@edcunited1878 posted kind of backs that up, that PSG and Bayern had taken UEFA's stance. That doesn't make them the good guys of course. It just means instead of pushing through a breakaway, they tried to bully UEFA into giving them more and more control. They got UEFA to propose a 50:50 joint venture of the CL commercial rights, between the big clubs and UEFA. The ESL clubs rejected, because they already had their own plans. In my opinion that proposal itself is outrageous, why should the big clubs have that much control? Reform UEFA, but let's not give elite clubs even more power.
I think I went a bit off on a tangent, but my speculation is that it's not just 50+1 that likely prevented this. I think there were other points that Bayern and the ESL teams didn't agree on.