Tall kids are normally good at headers because they get a lot of practice, it's easy if you've had a growth spurt and can get to every ball.
Haaland though was short until he was 15 and then grew 20cm/8 inches in 3 years. That's why people who coached him say his technique is so good, he was essentially Phil Foden sized all through youth football and had to play like it, and now he's 6ft 4.
Look at how small he is here compared to now. Mad. He's just average height here.