I'm reading too much into this sentence fragment and it fills me with fear.
I smell breaking domain changes in the future. They can't allow the .auth0.com tenants to operate as-is forever, which means existing tenants will get grandfathered in and eventually forced off the .auth0.com domain onto okta's domains.
I smell messy login sites in the future. I like Auth0's implementation of their Universal Login page, which didn't require JavaScript. In the quest for 'one single brand identity' someone will force a migration to Okta's login implementation instead.
That will come with changing client IDs, client secrets, M2Ms and everything else needed in their setup.
I might as well create a Jira ticket for this now.