By itself, nothing. However, most modern VPN solutions can require both user and device authentication, as well as various other host posture checks that typically can mitigate many identity-based compromises. Unless I haven't been keeping up, vanilla mTLS, not so much. While is may be more challenging to steal a digital certificate, it certainly isn't impossible if they aren't hardware-protected in a smartcard or TPM or some such, which really doesn't work for mobile and as such frequently makes it a non-starter. I've also seen some really poor mechanisms for provisioning client authentication certificates, which can mean any controls around protecting the certificate on the client device are moot.
Granted, traditional "full" VPNs have their own large set of issues, which of course is one reason many enterprises are moving to ZTNA-based solutions Bad guys are getting better, not only from a general maturity perspective, but increasing motivations for their upskilling - in the eCrime space there is plenty of money to be had and in the state actor space, our increasing reliance of (mostly) relatively fragile digital services increases both the military and political value of a potential breach.
Short version is that the "(user) identity is the new perimeter" can now be considered a dated concept, no longer sufficient, and increasingly less so. It is necessary to minimize one's attack surface, which cloud services mostly do pretty well by not exposing underlying infrastructure as is still common on many internal enterprise networks (of course, you are making the assumption they aren't exposing their cloud infrastructure to their own employees in such a manner). But also require more advanced authentication and continuous assessment (to look for things such as post-authentication token theft), which are... not so much natively, and beyond the capabilities of most 3rd party SSO solutions that work with said services.