I think that's reading too much into the Doctor's memory lapse about the GI. Remember, most viewers of the new series aren't familiar with the old series, and the new episodes have to be made to stand on their own.
In-universe, the Doctor is supposed to be over eleven hundred years old now, and was less than 450 when he encountered the Intelligence. How well do you remember things from nearly 2/3 of your lifetime ago? Even with a lifetime far shorter than the Doctor's?
I of course considered that, and it is perhaps the most obvious interpretation, but I'm just supposing that it may
be something more given the theme of memory running through the whole Moffat era to date, and this episode in particular; the memory worm, "run, you clever boy... and remember," and with the 50th coming up it would make sense for them to do something that harks back to the old days. It just felt
like foreshadowing to me.
Having the Doctor remember the details of his previous encounters with the Great Intelligence and explain them to somebody would've been too much of a distraction from the current story, so it had to be kept an Easter egg, a passing reference that would mean something to the old-school fans but not distract or confuse the modern viewers.
I don't think it would've been as it could have been done very quickly, as with the encounters with other classic foes. It didn't require any detailed synopsis of the GI's earlier appearances - something like "there are Cybermen in our universe" is all you'd need, like "I've fought it before... a long time ago. I didn't make the connection at first." That wouldn't distract or confuse the casual viewers (most of them are smart; they know it's a 50 year-old show - it's a mainstream British institution and the upcoming anniversary has been mentioned a fair bit in the press already - and would likely gather that it's a reference to an old story without it hampering their enjoyment) and would be a nice Easter Egg for the die-hards.
The "it rings a bell" was instead far too jarring
and distracting to this long-time fan (even if the intent was to simply suggest that the Doctor's getting on a bit, it runs the risk of being misinterpreted, hence this discussion). The Doctor has often talked of events from his past - including his childhood - with a degree of clarity. The Web of Fear
was the adventure that included his first meeting with a certain Col. Lethbridge-Stewart, and I'm not sure he'd just forget that, no matter how long ago it was now (if he is just forgetting these things due to old age, then he's in trouble, as a man is the sum of his memories, a Time Lord even more so...).