Quite some misinformation here. Filevault that secures access to the personal data stored on the device is not enabled in MacOS by default, by default anyone can boot recovery and reset the admin password to gain access and copy off all data. Once iCloud is connected with Find My to reset the admin password an iCloud login is then required. The device can still be booted up since the preboot filevault auth is not on, but then they are stuck at the logon screen. If your Mac was set to boot to a login screen where you first have to enter your Mac's user password then unless that password was set to 1234 it is unlikely anyone was able to access your personal data stored on the Mac.
But if you had it boot to the desktop directly without entering a password, which is entirely possible to select on MacOS with Filevault off, then they could have done that and copied all data. That would have been the only way where they would have had guaranteed access.
Furthermore, the Mac was likely wiped as it is sitting on the activation screen. Wiping is always possible with how the Mac security is designed. The device still remains useless and can't be activated, but deleting all your data is not something that Find My or Filevault protect against.
There is no recovery possible after such a wipe. It might have been cheaper purchasing another Macbook at your destination and theoretically you can't be 100% certain the device wasn't infected with some persistent Thunderbolt firmware exploit. At least Intel Macs in the past were susceptible to it, but of course with thiefs who just want to sell stolen wares they won't concern themselves with it. I am honestly just surprised they didn't smash the screen out of anger when they realized they won't be able to use it. Personally I still wouldn't trust the device as it could theoretically have had its hardware modified or some persistent malware installed and would replace it just as a precaution.