That is good context - that sounds like an implicit commitment to be working on that project. If a co-founder of mine bailed because they found something that sounds more fun, that doesn't seem like a good working relationship and would discourage me from ever working with them again.
Ah, I may have misunderstood - you are saying you would get a 15% co-founder stake, not that you already have 15%? In that case, no worries, you are good on either project.