IT Security Guy here.

ISIS is completely outmatched in any type of direct cyber warfare and will get drone striked in no time if they fuck around.

That said, an indirect type of cyber warfare against companies around the globe would allow ISIS to gain access to information or financial resources they could leverage to further their cause.

They won't be hacking into your local utilities but they might get access to information we'd rather our enemies not have...

Edit: McAfee is a kook btw, anything he says is likely influenced by the high amounts of synthetic amphetamines he's been heavily into in the past decade(s).