There was no need for you to nit pick. If it could have been done better or needed improvement, then at 12, that's something he should be assessing for himself or with the help. Your criticism is not helpful in any way. And your criticism was just critical - there was no thought behind it, no explanation of why you were criticising, it was only later, after posting here, that you thought of a "reasonable" explanation of why you were right. If it did need fixing, you could have helped him work out a better solution.
Your DH is right.
It's not about mindless praise, it's about recognising what they have done well, acknowledging the effort that has gone into doing it, feeling a sense of pride and achievement, helping them to assess their own work and identify if there are any improvements they could make or things they could do differently next time.
How would you like it if after serving up a meal, your DS or DH said "thanks for that but Nan makes it better"
Or a neighbour knocked on your door and said "The flowers in your garden don't look as good as Bill's"
Sometimes, there is simply no need to criticise.
You can push children without criticising them.
Yes if my child singing was so bad it was making my ears bleed I'd say maybe singing wasn't their thing and try work on something else!
It wouldn't occur to you to support them to improve?
What if it was Maths that they weren't good at? Would you tell them to give that up too?
You would be teaching them that if they aren't naturally good at something - best give up and do something else instead.