There is no need to validate any precise value that will change anyway. When averaging over 2016 blocks the result will get close to average mempool size that other nodes see.
You can't even measure the mempool size of older blocks for it to be close to the size or not, because as I said in my post above mempool is not stored anywhere. Even if you could, when calculating target we aren't looking for a number that is close to correct one, but we are looking for exact values.