IIRC, I mostly argued in favour of Malcom when he was been compared to Pulisic. Because things hasn't quite turned out well for him at Barcelona does not mean it would have been the case at every club, and besides, Pulisic just got his big move after falling completely off the cliff at Dortmund, so we will see how he does.
Tielemans has at least done enough to win numerous player of the year awards, even appearing in Europa team of the season and bagging 23 caps for Belgium so it isn't only hype. I don't know what could've gone wrong at Monaco, but he was impressive at Leicester in a league of higher standard than Ligue un even though 13 games might not be enough to fully judge. His performances in EPL could be a sign of his capability to turn it around and if he wasn't good enough, he'd have stuck out as the inferior player next to the likes Ndidi and Maddison, but didn't. Performing to a high standard in this Leicester team surely should be seen as a positive, not a negative.
Tielemans wouldn't be in the convo based on his time at Monaco after Anderlecht, but he probably did enough at Leicester to get himself back in it. Longstaff on the other hand wouldn't be in the convo if he wasn't British and relatively unknown. He has 9 top flight games at the age of 22, and hasn't failed anywhere because he's not being to many places. You could put down Tielemans based on his time at Monaco whilst ignoring his Anderlecht, Belgium and Leicester careers, but what's the point when you go on to big up Longstaff who's barely done anything at Newcastle. He looked decent next to the likes of Hayden, Perez, Clark, Ritchie, Lascelles, Rondon, and performed as well as them, but that could well be his level. He didn't stand out at Newcastle as someone like Zaha does at Palace for example, and he isn't the first player that have looked decent in the midst of a relegation scrap. If he isn't good enough, then he would stick out like a sore thumb next to Leicester City players, and even worse next to Manchester United players. Even though Pogba has not performed fully to expectations, but he still stands out as the best player in our team.
You could prefer Longstaff based on his technical skills or versatility, but that shouldn't be at the extent of putting down Tielemans when Longstaff is a nobody compared to the latter.