Comment by labcomputer

I tend to agree that this doesn't really add a second factor. After all, both the mTOTP secret and the password are something you know, and something you could tell someone else.

However, I do think there is added value here, at least in principle: It increases the difficulty of credential stuffing attacks, which, IMHO is the main value of having a TOTP secret stored in one or more password managers.

"Regular" TOTP stored in a password manager also helps with password reuse when a site is compromised because each site has a different TOTP secret. It seems implausible that a user could remember a different mTOTP secret for each site.