Possibly, but it seems a strange coincidence that Picard would order a ceasefire just as his weapons were about to become ineffective unbeknownst to him....
True. On the other hand, the Borg already had stated that 'We have analysed your defensive capabilities as being unable to withstand us.'. Suppose that the Borg had these awesome adaptive capabilities, that would render them practically invulnerable to whatever Federation weapon after the first few shots, making their statement generally true-- except that they'd be shot to hell first. The Borg wouldn't be very effective as an enemy if they would just ignore such inconvenient little facts in their analysis. Nor do I believe the Borg are
lying here - except for the Queen perhaps, I don't think the Borg lie.
Also, if the cube being destroyed was a real possibility at that point in time, why wouldn't the Borg have taken immediate action instead of just sitting there 'in stasis' and 'regenerating'? At the very least they could have fled to a safe distance. Did they already learn that the Federation wouldn't be that aggressive?
Would have been a very different episode though.
<Enterprise blows Cube to kingdom come>
Picard: OK, Q, you see? We
are ready for anything out there! Now I will exercise my right to gloat over you and pontificate some more!
<end episode>
I'm more inclined to base this on what we later saw in Voyager, that for the most part the Delta Quadrant was not commonly populated by vessels that could put out the sort of firepower a galaxy class could, so cubes were simply not built to accommodate it. The phasers blew a galaxy class sized hole in the cube, but require several shots to seriously damage a BoP. The implication for me is that Borg vessels simply aren't engineered to be all that robust at this point.
I don't think an agressive civilization would build vessels based on the capabilities of other vessels your region is 'commonly populated' with, but rather build vessels that can withstand the strongest enemies you'd reasonably expect to encounter. And we have seen that there are a few races out there that seem technologically superior to the Federation, some of which have had confirmed contact with the Borg for a long time (Arcturis' species, for example, that were able to hold off the Borg for a long time. Somehow, I don't buy that his race would have superior sensors, intelligence, particle synthesis, and slipstream among other things, while still having inferior weapons and defense systems. Not impossible though, as we have seens species such as the Satarrans in TNG.)
But at the end of the day, all this is of course nothing more than trying to reconcile a bag of mixed information created on the run, and at different times by different writers who just had to come up with a good story in time ....