That's not a statement about 1Password; it's about the fact that the security models are different on the two platforms, and I'm very familiar with how 1Password works on macOS and less so on Windows.
https://msdn.microsoft.com/en-us/library/ms995355.aspx
At that point you should probably be about as (in)secure as access to the platform is. I don't know how you could improve much on that (assuming secureboot and bitlocker encrypted disk).
Is there some magic going on the MacOS side that somehow improves on this?
The hard problem is getting the passwords out of the encrypted store and into form fields in your browser.
They also support copying the password to your clipboard (which they then clear after a few seconds). There's also the automated entry system which basically emulates keystrokes.
[1]: https://www.microsoft.com/en-us/store/p/1password-alpha/9nbl...
1P4 for Windows was the last version that was "buy once and forever", but they weren't providing good browser integrations for that version.
I am happy to support them though, and gladly used their products.
I definitely don't want to have to unlock my vaults on their website though.
1Password 4 for Windows uses local vaults just fine - I'm using it right now. The new 1Password 6 for Windows does not support local vaults.