The wealth taxes being talked about, broadly, would impact people with wealth of tens of millions of dollars and above. But PG talks about what they would "mean in practice for a startup founder." Already he's made a clever rhetorical move and conflated startup founders (a large group) with very successful startup founder (a much much smaller group), in attempting to broaden the people who look like they're impacted directly by his argument.

Over the medium-long term, I think any countries that don't implement a wealth tax (or some other way of addressing wealth inequality) will not remain developed countries.

PG uses 'stock' throughout the essay as opposed to 'wealth', and I believe he did so on purpose; he's appealing to emotion. Startup founders care a great deal about control and ownership, more so than wealth in the abstract. Look later as well - "And at 5% this threshold is getting asymptotically close to being an upper bound on how much of the company you get to keep." - he's directly talking about ownership/control rather than simple wealth.

Yes, this is obviously true. But again, PG's phrasing here is key; he's making it seem like you'll have only 55% of what you start with, which isn't remotely true. It just means your wealth compounds at a lower rate. Much less impactful, clearly.

Maybe. A wealth tax on its own doesn't solve this, you also need to sort out tax havens. And again, you need to look at the real cost of these people leaving - if a billionaire moves away (instead of paying a wealth tax) what does that cost us?