He's arguably right, and it may seem foolish for Trump to have expended so much effort on this issue when there was always a real chance of losing. Moreover, there is no easy "fix" without a constitutional amendement, which is not going to happen.
That said, the 14th Amendment per se really needs some interpretation to gain either result, since the writers clearly were NOT anticipating the sort of NGO-sponsored "invasion" that we actually have.
Still, it may be for the best that Trump lost here.
Winning would have provoked violent outrage, and achieved little.
The simple fact is that if you just DON'T HAVE illegal aliens, then you don't have a problem with their kids.
So make sure their numbers go way down.
Make more laws against birth tourism and enforce them aggresively (lifetime bans and seizure of assets, etc).
And ensure that any resident child who is an American citizen grows up IDENTIFYING as American and understanding what that means.
These are all achievable.