This sounds like FUD
Why would a spy chief go on record to say "we can't spy any more"? That sounds counter-productive. So the obvious answer is that they can and do spy, ok maybe it's a bit harder, but it goes on nevertheless.
Maybe he is saying "now that you know we used to illegally spy on you, we can't do it". Well, you know, touch shit. The law applies to GCHQ as much as it applies to me - if I tried to wire tap GCHQ they would be down on me like a ton of bricks, so why not vice versa (hint: bring it on, I do actually have very little to hide!)
So, in all of this, I am reminded of the late Mandy Rice Davis - "He would say that, wouldn't he"