Why would the batteries be going into E-Bikes if they are in limited supply and it is more profitable to put them into high margin cars? Are batteries really in limited supply?

I don't believe the parent comment meant that those e-bikes are the luxury products. Rather that e-cars tend towards the luxury segment. This seems objectively true when you look at the “first effort” electric vehicles from various manufacturers tending to be technological marvels with all the latest bells and whistles, rather than focusing on affordability out of the gates.

I think you missed the rhetorical point of the comment. The OP said that batteries weren't used in cheap cars because there were incentives to direct the scarce resource towards luxury cars where margins are higher. Then went on to say that battery-powered bikes sell like hot-cakes. The reply was questioning this logic - why would the scarce resource be directed towards even lower-end devices if the resource is that scarce. Then explicitly questions the assumption about whether the resource is really limited.

Of course, one answer could be that e-bikes don't use the same types of batteries that cars do, or that one could make 50 e-bikes for less than the battery array required for a low-end car. Another answer could be that the margins on luxury doo-dads are extraordinarily high, and it's the quickest way to recoup the R&D expenses invested in first version EV's - it's nothing to do with batteries being a limited resource.

Also, there's the idea that if a customer invests in a futuristic vehicle (EV), it should reflect that in all aspects of the vehicle (touchscreen, cameras, all the electronic things). This may or may not be valid, as personally I find myself inclined in the opposite direction, but I'm also not someone who rushes to buy latest tech anymore (so I may be an entirely invalid datapoint insofar as car buying market is concerned).

they're different batteries, and also a car manufacturer can't stop a bike manufacturer from making e-bikes with their own batteries, but they can decide for themselves that their batteries go to higher margin products