Getting username by userID without EBS

No.

This example describes a method to do what you are after

The JWT from the extension is passed to the EBS
The EBS generates an App Access Token/Client Credentials token
And uses the UserID from the JWT + the App Access Token to lookup the user on the API.

TLDR: you need an EBS

You mis intrepreted what I wrote on the thread you necro’ed

Use of kraken would likely result in a deny, because it’s use is deprecated.

And you can’t call helix without an access token that you can’t generate in an extension front end to use with the API