Hound - I believe the Hermetic code is based in large part on Germanic law and the idea of wergild - the payment of monies for the loss of life or capacity.
In context, the quote is as follows, in the context of Opening the Arts:
In the scenario you describe (with the grogs), the full legal scenario is that the property that has been taken from you may be demanded back (along with maybe a fine of some sort), or failing that, have wergild paid in its stead.
The same scenario applies to a Gifted servant/hanger-on: you may demand them back, or failing that may have wergild paid in its stead.
Except that the apprenticeship clause above explicitly states that you CAN'T demand them back. If someone took a Gifted lab assistant, and used them only as a lab assistant? I would agree with you. The scenario you describe is valid. But once they become an apprentice, the legal relationship changes - they are not bound to the original magus (and any legal obligation the original paren may have made on their behalf), but instead to their paren.
I'm pretty sure that's what "legally bound" means - in both directions. The Magus has the property of the apprentice, but the apprentice no longer belongs to anyone else, either. And if the legal bonds associated with "this was my biological child" can't survive the apprenticeship agreement, then a mere contract likely wouldn't be able to, either, regardless of how much magical power is implied. Otherwise, every single paren that has a Gifted child would set up a similar scenario - get together with a sodales, agree to a bajllion vis in exchange for the child at some later date, and then reneg on the agreement when they actually open up their own child's Arts.
Basically, your scenario sets up a sort of Golden Parachute scenario, whereby finding a Gifted child means that a magi could put some sort of legal contract on them, making them invulnerable to apprentice-snatching. "I was going to sell that lab assistant for a five queens of vis. You have deprived me of that power!" Or something sufficiently high. And if you have a sodales that actually DOES have five queens of vis in reserve, they can act as your legal justification to keep a Gifted lab assistant for as long as you want. But from the Code's perspective, apprentice-snatching explicitly breaks any sort of legal bond the apprentice had.
Therefore, you'd have to argue that you can't demand them back, but you can still demand wergild. Which is prima facia a non-starter, IMO: wergild in place of 0 is 0.