Need help with equity ask

So, I have this offer from one of my seniors to join their startup. They haven’t yet built the product. It’s just in ideation phase. He had successfully built startups before and also sold one of them. It’s not a tech product per se but most of the work would be on the business side. He’s offering 5% equity to look at their growth and marketing side(no salary). They are keeping 10% for employee pool. He mentioned that he would be initially putting funds along with his co-founder.

He mentioned that since he has connects and he would be able to get investments and businesses, the number is fair game.

I had asked somewhere between 12-15%, but they told they can’t give double digit equity under any condition.

What should be my take here?