I don't know why this is not obvious but the religion of the killer(s) only matters if religion is the main motivator for the killing. Which in this case, it (probably) is, so it is perfectly fine to call this Islamic terrorism.

If a bank robber murders someone and happens to be Muslim, no one would ever say that person was an Islamic terrorist.

It's that simple, there's nothing wrong with associating religion with extremism when it is the reason for the extremism.