That doesn't account for the new shares being (presumably) preferred while the purchased stock are common stock.

If there are no new shares, it's hard to prefer them.

You can have no net new shares while still swapping common for preferred. I don’t know if that’s what’s happening here.

When you do a private tender offer like this, either the company or the new investors purchase common stock from employees. It's possible Stripe is issuing some preferred shares to investors as part of the transaction, but that is being offset by it acquiring common stock from employees (eg: issue 100 preferred, buy back 100 common. Net result: 0 dilution to the cap table).

If they are swapping common out for preferred, and I don’t know that they are, the functional result is dilution even if the number of shares stays the same.

That's not true. Dilution is based on the number of shares, not the price.

Common vs preferred stock only matters in the event of same or liquidation if a company - preferred is paid out first. At IPO, both kinds of shares convert into the same public class of stock (with the rare exception of founders creating a special class of shares for increased voting power).

They do both typically convert to the same class at IPO, but preferred often has a conversion ratio — that is, it isn’t always 1:1, the way common is.

Functionally dilution should be considered in the full context of the financial outcome unless we are talking about voting rights and I wasn’t. For small shareholders the only real value of stock is money. If it is worth less money then its value has been diluted.