My account was banned permanently for account theft. I play on PC and just use the app for...

Signing into accounts that you are not the owner of can lead to permanent bans for account theft/fraud. Even if you are given permission by the account owner, you can be banned permanently.

Using the transfer tool to gain access to games that you have not paid for will be seen as marketplace theft/fraud and can lead to permanent account bans as well as possible permanent console bans. It happens all the time.

What you're doing is actually Marketplace Theft and can get your account permanently banned. That is the same story EVERYONE uses to try and get a free gamertag change. I recommend you stop to avoid a possible permanent ban.

What sort of enforcement can I expect for marketplace theft?

If you involve yourself with marketplace theft, your account can be permanently suspended with repeat violations possibly leading to a console ban.

Unfortunately for you, we know the story you're using. This is called Social Engineering and can lead to a permanent account ban for Marketplace Theft. I recommend you do not follow that tutorial.