It sounds to me like you are getting worked. The way you have been approached is not how someone with good intentions does it.

Reject the "joint launch" proposal. Put together a Memorandum of Understanding with a break up fee that will cover you.

If they are serious, they'll take you up on this and if nothing red flags they'll buy you.

If they aren't serious, you'll see them show their teeth pretty quickly as you are now denying them what they sought for free.

I suspect you'll see their teeth. In that case, launch your thing, get traction and force them back to the table.