Possible, but highly unlikely.
For starters, if that many were killed, I think it’d be really hard to keep a lid on it these days.

Secondly, I’ve done a lot of reading on the atrocities of WW2, and two immediate constraints come to mind:

• Bodies. While possible, disposing of that many bodies without anyone noticing will be really hard.
• The gas chambers of ww2 were only planned during the Wannsee conference in 1942, long after the holocaust actually started. The gas chambers were built because the original plan was not sustainable: People killing other people at an industrial scale. No matter how vile a person is, the human mind was not built to deal with having to mow down swathes of humans with a gun. Even the worst SS troops largely couldn’t deal with killing that many defenseless people, even if they were convinced it was “for the greater good”. The amount of SS troops who ended up unable to continue due to the mental after effects is staggering.