I think I know where this is going,it’s difficult even for Muslims to agree what he said,oral traditions and all,there some things that don’t quite add up,there have been more than 73 sects since he was around so there’s a possibility it’s not,like many Hadith,it doesn’t really matter to me.
If it doesn't really matter to you, dont quote it. If you dont believe its true, dont quote it.
I know that quoting things to attack some theology picked up off the internet is a habitual thing. But try not to do that.
Just for your information, the hadith about 73 sects is having a mid chain narration that Muslims call thadhlees. I dont think you really care about these things but I will say that anyway. So the bottomline is these ahadith that has that kind of narration is not considered Sarih. (I said Sarih, not Sahih). Many atheists dont know any of this but just blurt out things like "muslims believe it" just to create an argument but they really dont know what they are doing. I would request you to grow beyond that kind of polemics. These are tricks played by someone else and you are repeating it. I can see you have honest intentions but dont blindly follow others.
A thadhlees is done by what traditional muslims call a Mudhallith who makes up chains when he doesn't know who narrated it. So though some muslims believe these things, scholars have nuances they consider. Also what you should know is that the number seven in Arabic is Sabah and is used like a "number". So when people say numbers like 73, it is not necessarily a specific number. It could also be to say "a large number".
There is also the Islamic concept of Taaweel al masaara which means understanding of things like this will grow with time and place.
Bottomline is, its better not to just make some comment you have heard of on the internet. I thought I will tell you this because maybe you are a person who can think.
Cheers.