Because intelligence is general and correlates well across fields, and intelligence has a loose causal link with a PhD in biology. So I wouldn’t say it "helps" with investment per se but there is a connection there.
A very small connection. Investments aren't exactly just intuitive knowledge one can grasp just by being "smart". It takes specialized training to know what you're doing – and even among those who study finance, you won't find consensus on any single investment.
To be fair, a PhD in biology likely makes the individual more savvy when it comes to certain classes of investments (e.g. BioTech startups) than most full-time investors... so the criticism cuts both ways. Case in point: Theranos.
Ben Carson.
One counterexample doesn’t make a point. I could list many people who are simultaneous experts in multiple fields and it wouldn’t help my argument either.