how to sign in only using the username

You can't sign in with only a username. Otherwise anyone could steal your account.

I have already tried using this - it only reveals 2 letters of my original username though. Is there a way to sign in to that account using the password from the recovery tool?

There's no way to sign in using only the gamertag. He would have to find his Microsoft account and then he can reset the password.

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The Differences Between Gamertag vs Username

With the upcoming migration for Java Edition, there may be some confusion about what Gamertags are and how they differ from Usernames. This article should clear up any confusion about the differences between the two.

Gamertags are used for Microsoft accounts only. If you view your Microsoft account online, you will see your Gamertag as your account name and not your Minecraft Username. Any purchase information and account settings tied to your Microsoft account will display your Gamertag. You can find your Gamertag by signing into https://www.xbox.com/en-US/ and checking the top right.

When playing Minecraft: Java Edition you will see your Username in-game and not your Gamertag. The only time you will see your Minecraft Username is in-game and in your Minecraft account settings. You will still be able to view and change your Minecraft: Java Edition Username as usual on www.minecraft.net. Just navigate to your profile information.

https://help.minecraft.net/hc/en-us/articles/36...

Simply sign in using your username and password for microsoft on your console.