Their point is totally valid. There is no way reliable way to know who DOESN'T open your emails. You can get a list of who definitely opened your email AND said "OK to loading images and/or clicked on a link in the email. But if they don't load images and/or don't click on a link, you have no idea if they opened your email or not. Maybe they're a privacy nut, so they're read your email then type in your website's URL into your browser instead of clicking the link in your email (because they know you or someone is tracking who clicks on what).
Assuming that it is true that Google is NOT linking the two together, I see no reason to delete subscribers that APPEAR inactive.
