In terms of security, this is actually a rare case, unless a specific feature like fingerprint recognition isn't implemented via a web browser. The problem is that if 2FA is set up via an app, users must download it from the Play Store or App Store, which may be restricted in some regions.
I think, our main goal should be whether 1win Casino can be used without any problems. And I'm not worried about security as I don't see any issues so far. I believe it, security is much stronger when 2FA is attached to any account. I think 1win team has no system to add two factor authenticator. Even I looked in playstore and couldn't find any app. Although there were some visuals but it seems to me that it is not 1win real app. Avoid downloading any kind of fack app. Otherwise, you may become a victim of scam.
If I understand you correctly, then I would say that 2fa doesn't work with apps alone, two factor authentication can be implemented on a site as well, if you own and or manage a casino, you mustn't have your own app listed on the playstore to build a two facore authentication system into your platform, a normal website can have 2fa implemented for the security of those using that site.
For example, Stake is one of the top casinos, they do not have any app yet but the site is fully secured with a two factor authentication system, this forum as well is a site and some several months back, 2fa was implemented here as well. Every site that really care so much about security must have a 2fa system implemented on the site.