I believe they had a lot more than they claimed, but not in the millions. I believe that if it were in the millions it would be impossible to hide. The fact is that despite China's ban on reporting, information still trickles out. You can't stop 1.4 billion people from sharing information.
As for lying about the numbers, to an extent it can be fudged. Since 90% of fatalities are the elderly, they can always attribute the cause of death to natural aging, cardiac etc. Even here, I'm seeing names of elderly that have clearly succumbed to Corona R"L, but they make no mention of it. All they really need to account for are the young, which make up a small percentage of fatalities. I can see them hiding a few hundred thousand cases, but not millions. Though I've been wrong before, and I can be wrong again.