>product idea, business plan and marketing strategy are already done

If they say this, their failure probability is too damn high. What many first time founders don't understand is that startup is a search process in the outlandishly uncertain world. You can't "do" the product idea and strategy before you iterated on your initial hypothesis and verified/discarded/revised it.

This is like startups 101, explained in the basic level entrepreneurship courses on edx/coursera and founder must read books like "lean marketing for startups" and "startup's owner manual".

If a business-side guy or girl doesn't know that, disregard them.