As in the summary - there is not much to describe, really.
https://community.bistudio.com/wiki/allTurrets
Both variants (with true or false in 2nd argument) of the scripting command do not return [-1] - the driver's turret containing for example a smokescreen in speedboat.
so essentially you want allTurrets for a transport vehicle always return -1 since driver turret is always present?
I would like it to return an array of arrays containing every possible vehicle turret — and that includes driver's turret.
Do you think this is a bad idea? Well, the workaround is simple enough; I can always add [-1] to the array if I wish to have it there.
Technically driver turret is not really a turret, but as you said workaround is simple enough to not bother with details:
{...} forEach -1 + allTurrets tank;