The help page on deleted questions on (English) stackoverflow states:
Questions that have been closed within the past 48 hours cannot be deleted, so as to allow for editing and possible reopening.
The context here appears to be votes to delete, rather than other types of deletion (such as deletion by the user or by a moderator, or auto-deletion of spam/offensive posts).
However, buried in the FAQ that is linked from near the very end of that help page, it states:
Users with reputation ≥ 20k (more precisely, the trusted user privilege; 4k on beta sites), can vote to delete closed questions within 48 hours of being closed, so long as they score −3 or lower.
This information from the FAQ appears to be correct, in that I am aware of a question that was deleted within about an hour of closure.
I accept that the help page might not be the place to go into full detail, but even so, it should not be misleading, and "cannot be deleted" is evidently an over-simplification (as clearly they can, under certain circumstances). Would it make sense to amend it to a slightly more nuanced version, such as "cannot be deleted except by trusted users"? Even just "cannot usually be deleted" could work if the aim is to be as concise as possible -- but it needs something to alert the reader to the fact that "cannot be deleted" is not the whole story.
(By the way, the question which I know was indeed deleted within a short time of closure is one that I had answered, so it is still visible to me via my deleted answers, along with the approximate timings of closure and deletion. However, I won't identify the individual question, so that this is not misinterpreted as an attempt to challenge that deletion.)