This are unbelievable double standards and you're not even aware of them, there is nothing that makes it happen in an action movie and doesn't make it happen in an action kid's game.
It doesn't matter as this machine is proven to explode with damage as shown when you destroy the core and many mini explosions happen on its own, followed by a bigger explosion. "A car for example" doesn't have this at all, nor any other machine.
You have no reason to conclude that because your inner logic about it is that it shouldn't happen and yet it did happen, you don't qualify it in any valid way.
Its machinery is made in a way that what happened makes sense, you can't dismiss it as not making sense or that the nucleus suicided, you are even aware that in fiction machines can blow up via stupid reasons like that and this is an alien machine with unknown properties.
What you say makes as much sense as trying to dismiss all the explosions because they happened in space, which has no air.
Everything follows cause and consequence, I don't think this means what you think it means.
You are literally wrong, white blasts came before the core blew up and white blasts came before Nova itself blew up. The latter white blasts come from the center of Nova, whereas Marx was thrown in the center of that hole Nova had on top of its face.
More to it, seeing the feat again I noticed that when Nova blew up there were mini explosion on top of the big & more notable explosion, those being parts of Nova that survived and were blowing up on their own.
It should have all been too intuitive the first time I pointed things out, you didn't keep up. Kirby can't cause many mini explosions and later a bigger explosion by destroying something, Kirby can destroy something that something blows up naturally by the sheer force of Kirby's last attack. This is Nova being made in a way that allows itself to blow up like that, which cannot be attributed to Kirby.
If Kirby could "one-shot" Nova as he did he could have destroyed both "the rest of the machine with 5-A durability" & the defensive system at once in 1 or 2 attacks since he can blow up all of Nova's body by your logic.
It wouldn't blow up again because it healed from the damage. It partially pertains the previous damage, if it were to keep "all" the previous damage it wouldn't have regenerated, its parts would still be separated and floating in space without gathering and repairing themselves.
You can have 2 characters with Mid-High regen and have 1 not die from damage in its core & blowing up and the other die from getting defeated in battle 3 times, being ripped in half and then blowing up.