Here is some of my own experience with my son.

From what I read, children's understanding of numbers go from concrete to abstract. This is a critical process of brain development and could take a long time. Here is some of the exprience I had with my son.

He is about the same age as your D. I am not a 推妈 at all, but his preschool has multi-age groups and he often picks up things from the older children.

He also does math with finger counting, or object counting. Once I noticed he brought back home some math work which was probably assigned to the older kids in his class, but for some reason he also tried. His answers were very interesting. For example:

2+3=2
5+2+4=3
etc.

What happened was he was counting each number as one object. So two numbers together is 2 and three numbers together is 3.

My son by far doesn't know as much as your D does, although I already consider him very advanced. I think we should give them enough time to go through the process of "concrete-to-abstract". It will be very helpful in the long run.