>that people will take code - put it under a different licence and modify it
Ok, although the 'take code and put it under a different license' is untrue, you can't re-license code unless you are the copyright owner, I gather that he means someone making modifications/enhancements and placing them under a license which OpenBSD can't use while remaining fully BSD licensed.
This happened when Apple adopted pf they added a few enhancements of their own but wrapped them in Apple's license so pf maintainers could not add the changes back into upstream pf.
This also happened with Linux's Atheros drivers (which were derived from OpenBSD, but modifications were published under the GPL instead of the BSD or ISC licenses; Theo argued that doing so was actually a violation of the BSD license (apparently they were stripping out the BSD license in the actual source files? I'm fuzzy on the details)). I don't remember what the outcome was.
Ok, although the 'take code and put it under a different license' is untrue, you can't re-license code unless you are the copyright owner, I gather that he means someone making modifications/enhancements and placing them under a license which OpenBSD can't use while remaining fully BSD licensed.