Maybe, maybe not. It doesn't matter one iota - it's illegal for them not to go, if they are compulsory school age.
It doesn't matter if they're ready for school - they have to be there
It doesn't matter if the soiling is due to SN or abuse or laziness - that child STILL needs to be changed, in a timeframe that doesn't allow for parents to be called and arrive at the school. 30 minutes is too long.
It doesn't matter who you feel passionately is at fault here, or what age people would expect a child to be done, or how indignant you are that this falls to the (I agree, already stretched and overworked) teaching staff - it's the child that matters at the end of the day. Leaving them in soiled clothes is neglect - and if a school allows it, they are rightfully guilty of it. So yes, teachers / TA's / general school staff absolutely should be the ones to change children if they aren't potty trained.