Um, i think that was the old hardlimit for blocks. Some miners did not update that yet. It's no proof that all transactions were cleared at that point in time. It's fully up to the miner how many transactions he want to put into a block.
That's not the hard limit. The 'hard limit' is 1 MB, you're talking about a 750kb soft limit. It is interesting because Antpool was one of those that mined a 749kb block, yet the block that they have previously mined was 930kb.
I know it is the softlimit but i remember a discussion where it was said that the standard setting in nodes was sat to 750kb as the hard limit even though the blockchain allower 1mb blocks already. Not all nodes had changed that and people asked them to adjust that value in order to help the network. But it might be that still some miners use that old hardlimit.
I might remember wrongly though.
It would be convenient to check the last blocks and found one of these blocks with only one transaction, the block reward, and believing that it shows the network is greatly underloaded.
If I'm correct that is because of SPV mining.
You are correct with that. Only that it means that it is fully up to the miner how many transactions he put into a block. It can't be judged from the blocksize how many transactions could not be included.
But since you mention SPV mining. I believe the newest implementation was that, when a new block was found in the system, the miners start mining instantly with an empty block. in order to not lose hashingpower. Then it starts to check transactions and slowly put the transactions into the block, starting to hash on the block with the additional transactions.
That might be the reason why antpool creates such blocks? Dunno.
..... Makes you wonder, why we seeing these high revenue, when transaction volumes are declining.