yes but the parents are engaging with your child, I doubt they are not engaging with their own, just pointing it out to you.
Pretty sure they were interacting with my DC as she spoke and she got obvious feedback - I also think she was engaging my DC to get my attention - which she got as she was very nice to talk to though strangely oblivious IMO to her own DC.
I had 3 under 4s so I'm aware how sanity saving toddler groups can be. I also had a toddler when we lived in another area - I never saw this behaviour there even though parents spent time talking to each other.
TBH when we moved here, it's a heavily deprived area within it, I thought the children centres pretty patronising - all the little talks the play leaders gave about playing well with your DC and the insistence chairs were out and we all had to be on the floor with the DC at their eye level so we'd interact. I thought it completely unnecessary but after a while I could see where they were coming from.
I suspect that you've just never seen this behaviour. ( I also don't think it the parents fault - it's some how their normal)
Perhaps, instead of listening to the parent complaining for two years someone at the school, the teacher or someone could have reached out to that parent and said, can we help you, to help that child learn his tables?
Could well be dismemberment - but I and other parents have offered many suggestions about web pages for the time tables - written them out offered to lent a CD which helped my DC - not constantly but when appropriate. I not actually prepare to go and teach her DC for her- I spend to dam long doing that kind of thing for my own.
It not just this parent - there lot like this - that one example.