The other guy was banned from the 400 subscriber account, the login of which he used to access his 250,000 subscriber account which was unaffected by the corrective action aside from him not being able to access it. He had no access to his channel.
He went through something with google services to shift ownership of the channel which is how he changed the email on file and gained access before youtube ultimately reversed their decision about the ban.
I only mentioned monetization because I don’t know if youtube considers that only a monetized channel is eligible for brand management but these days who even knows what constitutes a brand, it’s all just a bunch of jargon used by people who enjoy to jarg on.