After you connect the internal drive of your old Mac to the new MBP and decrypt it with the FileVault recovery key, you can boot the new Mac from the old drive to encrypt it again, given that FileVault can only encrypt internal and external hard drives.
When you run the Terminal command to check the FileVault state, and it says "FileVault: Yes(Unlocked)," it means the drive is FileVault-encrypted, it is unlocked now because you are logged into your Mac, and you can access the data. For a FileVault-disabled drive, it reads "FileVault: No."
If you boot up into macOS recovery mode and check the disk info in the Terminal, you will find that it is "FileVault: Yes(Locked)" given that the drive is unmounted now.
If you want to encrypt system or non-system volumes with FileVault, or decrypt FileVault drive, you can also try iBoysoft DiskGeeker, an all-in-one disk management tool for Mac.