Oh poppycock. It wasn't until well after the battle from second hand observations of the Germans boasting about how crappy the British shells were that it was known how bad they truly were. And if you believe during even a lengthy battle as this that it was determined that somehow Xlbs of concrete rather than Xlbs of AP shell would be better I've a bridge to sell you.
I'm not saying that. What I am saying is "gunners were seeing their shells were hitting but not doing anything, so they thought something along the lines of "Fuck it, lets use the concrete ones instead, they couldn't be any more useless than the normal ones!" and found out that they were rather more effective.
In a stand up fight, "smashing through armour with something that shouldn't be able to do that" beats "barely scratching armour with something that is supposed to get through" any day.