No one is surprised that Louis is writing but the thing is Louis said on AGT that he didn't have time to write songs because he was too busy being a dad bla-bla. Louis in the media: doesn't work because he has a baby and a gf VS Louis in real life: write songs. So that's why the whole situation is funny.
Oh it’s funny since there are people that believe the in the media version of Louis. But anyone with two eyes and a brain that doesn’t think the worst of Louis doesn’t actually believe he’s just been sitting around doing nothing all year.