A chip that produces refrigerator ram is also capable of producing hbm3 ? Don't they require retooling? Won't the same problems surface as required to establish new fabs?

They do require retooling and that's what's happening here. RAM manufacturers decided that it's way more lucrative to focus on HMB production than DDR 4/5 production. Capacity is the issue and that's capped unless you build new fabs but they won't do it because there's no guarantee that the demand will keep the same in the next years.