Normally the tools people use don’t dictate the license they must use for their code.
The unusual thing here is that the creators of a linker are apparently trying to have the copyleft licence propagate to code that is input to the linker. Others have pointed out that GCC has exceptions for this kind of thing, despite that it is released under a strong copyleft licence (GPLv3+).
Also, your account of copyleft is still incorrect. It's true of the GPLv2 and GPLv3 licences but not true of all copyleft licences. The AGPLv3 licence, which is the one relevant here, doesn't apply only on distribution.
[0] https://www.gnu.org/licenses/copyleft.en.html
edit I think I was mistaken in putting propagate to code that is input to the linker, though. As lokar's comment points out, it's instead about the output of the linker.