I think whether the problem is with the ISSL or the GPL is more of a philosophical question (I would be inclined to say the problem is with the ISSL and its restrictions on user freedom).
In any case, to be as specific as possible, for anyone reading this thread in the future, the problem is caused by the fact that the ISSL doesn’t allow modification nor makes the source of MKL available while the GPL requires derived work/combined works to be freely redistributable and modifiable by the user. See point 5 here: The GNU General Public License v3.0 - GNU Project - Free Software Foundation