IED just means “improved explosive device” as opposed to a manufactured device such as a mine. IEDs can even be manufactured devices used in unorthodox ways, such as using a cannon shell warhead in a fixed emplacement. So, unless the bomber was specifically using, say, demolition charges and was trying to demolish the memorial, or was using a claymore to try to kill folks, then he used an IED.
You’re probably right, though, that the article uses IED specifically to raise the spectre of terrorism.
Actually it means Improvised Explosive Device,, as in homemade.
Improvised explosive device.