You read those people as apologists, I would say perhaps they're offering some comfort and indeed some facts.
Whilst they may well agree that it's not right that all schools don't offer 3 separate sciences, I don't see how their approach is any less helpful than saying 'yes that's wrong it should be improved, what a shame that your child can't go to private school'.
The OP here is concerned that 30 in a class may be a problem - people have said it usually isn't, and offered their experience in support of that. I cannot see how that's 'shutting down debate' or being an apologist for anything.
Once again - there have been things over the years I'm less happy about with my girls' schools, and things I think they do well. I do not go around saying either school is marvellous. But if someone said, say, I am worried about my dd starting a large-ish comp with mixed ability groups as I think she will fall behind/be bullied/get lost in the crowd, so I think we'll have to go private, then for me to say 'well, actually, it may well not be like that y;know, mine survived and are doing pretty well' - well, how is that wrong? I would see it the same as someone saying 'I'm shitting it about driving across France on my own and getting lost' and someone else saying 'I've done that, it was fine and not as hard as you think'.
No-one is saying there's never anything wrong with state schools - or indeed with private schools, it would seem. But if I'm 'lucky' with the schools my children go to and should acknowledge it, then an awful lot of people are 'luckier' than they think or admit to with the schools in their area, too.
Contrary to what is suggested, the vast majority of state schools are not divided along the lines of 'over-subscribed comp with awesome results in posh area' or 'terrible comp with knives and dual science'. Most schools are pretty mixed, just as most places are. I didn't have any 'luck' here - I didn't have any choice either. I could say I'm lucky in the things the schools do very well, I guess - but then I could equally say I'm unfortunate in the things they don't. Acknowledging and applauding the many very good things about state schools is just not a bad thing - I don't see how it could be.