The MMC themselves run Manbij and say there's no YPG. The SDF has a presence there with Asayis and Internal Security Forces, though.
and it's clearly a lie
"Clearly" is subjective. No it's not clear it's a lie. For me, it's true to a point if not a slight hyperbole.
but he has no motive to make the SDF smaller than they are.
Or maybe it's an estimate? Like everything in Syria?
An independent turkish study estimated it to be around 60,000-75,000 waaay back in 2017, before many of the military councils were established.