A curious case of non-array array-like access detected

This was detected while progressing with MDL-67114, so I'm creating it as subtask there because it's leading to php74 failures (not happening in older php versions).

In any case, the code looks just "wrong", or I've not been able to find an explanation to it, so it requires review and, if possible, amendment.

Code: https://github.com/moodle/moodle/blob/master/mod/forum/classes/task/send_user_digests.php#L202-L204

Apparently we are replacing the whole array by the (boolean) result of a capability check. So any future use of the original array becomes borked.

For your consideration, ciao