I think it’s very sad to see Mormons represented by people of this miserable caliber.
In answer to your original question, I think the apologists might be half right. Of course we understand the significance of an altar, but at the same time we feel that this significance is evidence of the believer’s own stupidity and/or gullibility.
Again, Mormons are taught that other faiths are corrupt, and this idea of sacred space is one of the pieces of evidence of this teaching, along with paid clergy, iconography, statues, etc.
The defiling of the altar serves two simultaneous psychological purposes. It makes a mockery of the false religion, which is thought to be led by a conspiracy of people with Satan, and it also defines the desecrators as superior and different. The people who made a mock human sacrifice did so because at heart, that is the way they see the catholic ceremonies, as akin to something like a Satanic human sacrifice. At the same time, their mocking assured them that the Catholic God has no power over them, as they were symbolically calling down lightning and nothing happened.
All in all, these are faith promoting exercises for Mormons.