Ideally the tech should have confirmed whether you had a backup before proceeding with the repair. However, If you know you need the data recovered, then you need to make absolutely sure the tech knows this before proceeding with the repair. An Apple tech is then supposed to assist you in sending the bad Logic Board (or drive) to an approved professional data recovery service to attempt data recovery. The recovery service will return the Logic Board (or drive) to the tech after they are finished so that the tech has the core to return to Apple. The tech actually has to alert Apple that data recovery is needed so that the tech is not penalized for returning the core part late. At least that is how it is supposed to work (and did for us when we had a defective storage device where the regular backup did not capture all the needed files). The tech is correct that it is impossible to get the bad part back now that it has been returned to Apple.
Things get tricky with SSDs. USB-C Macs get even more tricky since the SSDs are integrated into the Logic Board which requires a professional data recovery service to access the chips for the SSD directly (if possible) or in the case of a Mac with T2 security chip (2018+ models) it even requires the recovery service to attempt to "fix" the Logic Board so that they can decrypt the data stored on the SSD, otherwise the data is permanently lost.
Like @leroydouglas mentioned you should always have frequent and regular backups. In fact if the data is extremely important you probably should have more than one copy/backup.