DM's reaction to being 12 mins late

[Coffeeandtveasily]

My DH, my two children and I were due to meet my parents at a local restaurant today for lunch.
It's a very relaxed, family friendly restaurant. It was really quiet, with loads of available tables.
Due to one thing and another we turned up 12 minutes late. I messed my DM to say we were running late and would be there soon.
We saw them sitting at the window as we arrived. My Dad looked furious. My DM was looking at her phone with a face like thunder.
My DM said "Oh you're finally here! You're late!" I said "10 minutes late! We're here now."
They both had a big go at us, with my mum saying "It's lucky we weren't late or we'd have lost our table!" The restaurant was quiet, loads of available seating.
They then grumbled and made more passive aggressive comments.

I was so angry I couldn't speak. The only reason I didn't leave was because the kids were excited to be there.

I was looking forward to it after being ill and finally feeling a bit better but their attitude spolied it for me.

AIBU to think turning up 12 minutes late isn't that bad?!

Personally I'd have just had a drink with my DH while I waited rather than angrily looking out the window/at my phone.

Every case is different, it's true. I also have extensive personal experience with several dementia-related conditions, so I do have some idea what I'm talking about.

It sounds like this?

I just can't see their hardship? Are they always such grumpy bastards?

12 mins would not bother me or any of my circle of friends/family.

They were inside with ample opportunity to order food/drink, with each other, no pressure from feeling they were holding up the table. I literally can't see their problem.

Of course being habitually or deliberately late is rude, that doesn't seem remotely the case here, evidenced by your reassuring text.

Do they not get out much?

Personally I think anything less than 15 mins is fine, especially as you messaged. We have a close family member who thinks nothing of being an hour or so late, I find it so stressful that I avoid booking to go out and meet them in a restaurant.