The first one is an opinion. You can disagree with it, but that neither makes it 'true' nor 'untrue'. There are many popular names I like, and many I don't. For the ones I don't, clearly many people do think them 'lovely' to have chosen them. It's a highly subjective term. No statement of "that's a lovely name" can be true or untrue, as it's just someone's opinion. If it's a popular name, the opinion that it's 'lovely' is obviously a popular opinion, currently, whether we personally agree with it or not.
I agree there could be huge pockets of children with a certain name in some areas and hardly any in another but, nationally, popularity is diluted massively compared to what it was. That is true, it's reflected in the stats. What I then said, based on that, was there's every chance their Leo could be the only one in the class ('could', I didn't say he would or wouldn't be, I was just speaking generally based on the fact there are far less boys called Leo born now compared to boys called Jack in the 90s). That can be neither true nor untrue, as it's just hypothetical. We have no idea where OP lives and whether there will be lots of Leos or not (or whether they'll even use the name!).
I agree with the point you are making, I just found your "that's not really true" response to me odd, as I wasn't presenting the potential situation I was outlining as 'true', just what may or may not happen 🤷🏻♀️