This is what find my past say about redacted entries.
When the Register was taken, it was with the understanding that the records would remain closed for the duration of the lifetime of the individuals listed. In the case of censuses, this same understanding is enshrined in law, meaning that all of the individuals listed are unavailable to view for 100 years and one day after the census being taken. In the case of the 1939 Register, however, advances in how we redact have meant that rather than a blanket blackout on all the records, we're able to show you the records of people born 100 years ago or more.
As well as giving you access to the records of these individuals, we've set up a system that will scan the Register at regular intervals, opening the records of each individual as their date of birth becomes longer than 100 years ago. This means that new records will be available to explore frequently, as more and more people pass this threshold.
However I think they go through the death records to check. My mother was born in 1923 and died in 1992. She is visible on the 1939 so that would suggest that FindMyPast check death records against the 1939 register.
If I recall correctly full dates of birth are shown on death records from 1969. Dates of birth would therefore be used to assist FindMyPast to find the correct person.