Their position seems to have changed since I last checked. It was originally
"The ruling by the Information Commissioner that resulted in the 1911 Census for England and Wales being opened early does not apply to the 1921 Census because, unlike the 1911 Census, the 1921 Census was conducted under the 1920 Census Act, which is still in force and which contains a statutory prohibition on disclosure."
It now appears to have switched to
"The stated government position from the ONS is "its intention to release the entirety of the 1921 Census returns in 2022, in accordance with the non-statutory '100 year rule' which was adopted to reflect this undertaking of confidentiality".
It looks as though they intend to revoke the statutory prohibition. I hope I'm still around to see it.