I'm not entirely sure why this is getting viewed as a Celtic/Rangers thing (and I'm not really keen on further derailing this thread with it all, so I'll make this my last post on it).
First of all, I've been to plenty of big festivals/gigs in Glasgow and all over Scotland, and not one of them has had an element of football partisanship that I've seen. Generally people attending these things aren't thinking about football.
Secondly, you may well be right that the people waving Palestine flags were Celtic supporters, I don't know. I honestly can't see how it's relevant. People attending the gig wanted to protest something the band was doing, as far as I can tell it's the same as when people protested U2's tax affairs at Glastonbury a few years back.
Why didn't they remain outside the venue? Why should they, it's a festival, not a radiohead gig.
All the "being seen to support" stuff seems like it's close to the phrase "virtue signalling", and is something I'm not going to get into.
Put simply, there's been a history of support for Palestine within Glasgow for many years, I don't understand why this display is being characterised as about football. Maybe it's easier to pretend it's about that than actually about Radiohead's decision to play in Israel.