So the whole OpenBSD "ZOMG Linux stole our code!!!!1" thing has surfaced an interested argument, namely that when a file contains an offer of either BSD or GPL license to the recipient, that offer is a magically immutable legal document, and that modifying or removing it constitutes "re-licensing" the software and is forbidden.

Which leads to the following sequence:

\r\n
1. I receive such a file, which contains an offer of BSD or GPLv2.
\r\n
2. I accept the terms of GPLv2.
\r\n
3. I decide to give a copy to someone else.
\r\n
4. I am required to pass on the freedoms granted by GPLv2.
\r\n
5. I am required to pass on the full text of the license offer, including the text of all offered licenses if they were present in the offer.
\r\n
6. Because, according to the OpenBSD clan's argument, this offer is immutable by any licensee, I am also required to notify the recipient of the software that he or she may not remove or modify it.
\r\n

At which point I have -- by the alleged requirements of the "license" -- been forced into a breach of Section 6 of GPLv2.

The only logical conclusion here seems to be that, when a file contains a dual license offer of BSD and GPL, it's not legal for any licensee to distribute under GPL, because the alleged requirement to maintain the license offer would mean passing on a "further restriction" and thus would breach GPL's terms.

Is Theo de Raadt really of the opinion that this is the case?

Extend the argument to show that the file is also not legally distributable under BSD if Theo's argument is correct. This is left as an exercise for the reader.