After the USS Europa was rammed by the cloaked Klingon ship (Battle of the Binary Stars) though severely damaged its antimatter field was breached in order to blow up both itself and the Klingon vessel. That would seem to be consistent as a means to destroy a ship.
How could they when there are three examples on Discovery alone
In each, there was a specific, pressing reason for it to be done,
in the moment. It wasn't done preemptively,
as a matter of course, "just because" or "just in case"! Do you really not see the distinction? The former is something we've always known Starfleet to do. The latter is something we've never known them to do. You can think it's dumb, and from the standpoint of a military mind you
might be right, but that appears to be their policy. Consistently.
Such as? The original post that brought up "The Catwalk" dialogue had the question "Standard orders?" at the end. It did not exclude the possibility of a bluff. Yet, you've already concluded that it was all a bluff in your reply to another poster, excluding the possibility that it was not.
The context in "The Catwalk" suggested it was a bluff, because he had also told them the crew was dead, when really they were all hiding in the nacelle. His motivation was saving their lives, not destroying the ship at all costs. He just wanted the Takret to believe that. It was a clever ploy, and it worked.
But for the sake of argument, let's say it wasn't a bluff. The ship was (1)
already being actively occupied and exploited by the enemy, and also (2)
was Starfleet's most advanced prototype vessel. Those would be specific, pressing reasons to blow up the ship. The same ones that applied to the
Buran and the
Glenn, respectively.
In TMP, they were inside V'Ger, which was approaching Earth and intended to wipe out all life on the planet. That's also a specific, pressing reason. The same one that (in less extreme form) applied to the
Europa.
Once again, none of these conditions applied to the
Shenzhou. She was just useless old space junk as far as they were concerned. Of course, they turned out to be (somewhat) wrong about that. Just like a lot of the
other decisions they made turned out to be foolish, even though they
seemed to be the right ones at the time, from their point of view.
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MMoM