It's not illegal to not honor written offers, it's illegal to distribute copyrighted material in violation of it's license.
The offer of source code seems to be a way to facilitate the conveyance of source code through opt-in means separately from the object code rather than some legal trickery to create a user-licensee contract.
While the offer may indeed convey a licensee-user obligation, a compliant distribution would attach a license anyway, converting the user into a licensee and licensor to licensee in a recursive fashion
I wonder if lawyers specialize in this, it sounds very cool and not at all standard law, but somehow compatible with contract law
IANAL
On the shelves are three insulin pumps: one with a 5-year warranty, one at a bargain barrel price that comes with no warranty, and one accompanied by a written offer allowing you to obtain the source code (and, subject to the terms of the GPL, prepare your own derivative works) at no additional charge any time within the next three years.
Weighing your options, you go with pump #3. You write to the company asking for the GPL source. They say "nix". They're in breach.
This is not only possible but also prudent for a device which can also kill you.
The argument is over providing you the source code.