As an example: the FSF's position is that yes, if you dynamically link against a GPLd binary that all the code has to be GPLd, and if one wants to link against a copy-left library in a non copy left program the library has to use the LGPL. However, the LGPL predates dynamic linking. It's from the days of static linking where the (L)GPLd could would be distributed together in a single binary. In the world of dynamic linking that isn't true anymore.
A primary example is libreadline. It is (or was?) GPLd and the FSF use this a hammer to try and get other code to be GPL. However, this (as far as I know) was never actually litigated.
As another example: the Linux kernel is under GPL2, Linus makes it clear that closed source programs can use the syscall interface without a problem, but its not clear that if he didn't say this that they couldn't. He includes the note to clarify his position that they can for those that would be concerned. i.e. the argument would be that without the syscall note, one couldn't ship any hardware device (say an appliance you stick in your rack) with linux and proprietary software. My gut is that most would find that difficult to swallow.
As an aside: I personally feel this has similarity to the google / oracle java lawsuit. If one thinks that APIs cannot be copyrighted then arguably anything that just uses APIs and doesn't embed the actual GPLd code one should be fine as the APIs aren't able to be copyrighted. If on the other hand one views APIs are able to copyrighted as well, then it would be much more clear that the FSF's position is correct. Though the usage might still be fair use (and to an extent one might argue that its even more fair use than Google's usage)
If this ever does get litigated, I'll be following closely as would be very interested (intellectually at least) in the result.
TLDR: until its actually litigated, everyone who claims to speak an absolute truth is speaking out of their ass. Lawyers are a conservative bunch in general so they'll give you the most conservative answer in terms of what you can do, that doesn't mean its the absolute truth in terms of what you can and cannot do.