In the eyes of 1D management any of them getting somebody (who is not a longterm gf) pregnant would have always been a huge scandal. Especially if that somebody was just a "normie" you mainly partied with after you broke up with your gf.
There are plenty of celebrity men that are rumoured to have secret children. It is probably quite hard to hide a child if both parents are celebrities (but not impossible).
But if the mother is a "normie" you could easily pay her a fortune to make her publicly disappear and/or link her to several men during the pregnancy. Barely anybody will remember her years later. And even if somebody says "Wait, she has a child that looks like the male celebrity she used to party with" - most people will pay no heed to it.
There. Was. No. Paternity. Test.
No multimillionaire celebrity who hooks up with some rando who then claims to be pregnant just says, “Okay! Cool. I guess I’m a dad!”
It’s not even like maybe he took one we don’t know about. They literally said he didn’t take one because “he trusted her.” It’s fucking ridiculous that people can’t put two and two together and see what a farce this is.