Currently USA used EA-18G, EA-6B and F-16C/D for SEAD. Hence US has plenty of SEAD capability.
Also SEAD does require opponent to use radars. It's generally useless versus optically guided systems (including Mk I eyeball) or infrared systems. As such NATO tactics are usually medium-to-high altitude to avoid these systems (mainly AAA and MANPADS).
Bare in mind ISIL/ISIS shouldn't have anything in its arsenal that can be used reliably at medium-high altitude. The Iraqis never had any such systems in service, whilst the Syrian did but these were mainly SAMs and mainly obsolete at that.
There's been no reports of medium range systems being used by jihadis in Syria – it's all been MANPADS and AAA and the Syrians are limiting casualties by using medium-high altitude (also limiting SyAF effectiveness due to lack of guided weapons).
I can forsee three possible explanations:
1. ISIL/ISIS got lucky. AAA is not reliable in hitting at medium to high altitude.
2. F-15 was flying at lower than standard altitude recommended by current tactics.
That's possible as there might be some confusion as to ISIL/ISIS capabilities, especially after over a decade of fighting much lower capability opponents in Iraq and Afghanistan.
3. ISIL/ISIS actually used a captured Syrian SAM ala SA-8 or SA-9 and then got lucky as well.
This one is possible but we've had no reports of medium-high altitude SAMs being used in Syria.