That's only true if you are using an online service as a password manager, so the master password is the only thing protecting you. Not necessarily for offline password managers. E.g. in my case, I use Keepass that I never sync/store online, so even without enabling a website's 2FA, for many attack models I am effectively using 2FA: logging into the website requires both something I have (a device with my Keepass database) and something I know (the password for my Keepass database). But without website 2FA those two factors then produce one single factor (the website's password) that is transmitted to log in, so enabling website's 2FA and storing it in Keepass makes it 2FA against even more attack models, i.e. attacks where it's not my password database that it compromised, but just that one password. So it's still a benefit.

If I ever feel the need to sync my Keepass database, e.g. on Dropbox; I could set a key file (that I transferred offline between my devices) in addition to the master password to preserve this 2FA aspect, so that even if my Dropbox password and Keepass master password were both compromised, they would still be useless without access to my devices that contain the key file. But I never had the need to use my password manager on a different device, so no syncing needed so far. In any case, I don't actually care about 2FA (when I enable 2FA, I actually do it to decrease security, not increase it, as I explained in my other comment), this 2FA is just a bonus of my not needing and liking online services.