Well, you can currently, but you won't be able to in the future when Vanguard is actually enabled. The modal here just doesn't check for Secure Boot.
League won't require Secure Boot when Vanguard is enabled. That's why the modal doesn't check for it. :)
It's required on VAL but they're solving different problems for us, and Secure Boot is less critical to the problems we see Vanguard solving for League, so we don't require it.
TPM2.0 will need to be enabled though if you're running Windows 11.