[RPG] Do people get XP for battles they weren’t in

Colossal armor is not going to fix it

Colossal armor is Spectacle, but once the idea has worn off the combat will be exactly the same. The PC's abilities to become giant and fight other giants will not really offer any difference in the way they fight. They could pick up and throw a building, but they could do the same thing on a smaller scale with a rock. Now if they were fighting giant, armor clad enemies as regular sized people things could certainly get interesting.

"Terrain is often of more value than bravery....Bravery is of more value than numbers." - Sun Tzu

Your original instinct to use terrain was right, terrain can and should shape every combat that occurs. If the party isn't using the terrain, the enemies certainly should. Anything with the intelligence level of a wild predator (like a wolf) or higher should be able to utilize the terrain in some way, sentient creatures and people should especially make use of it to gain advantage in combat. For example if your players are able to flank enemies that were behind cover those enemies should probably be Shaken as their safe position is now exposed.

Combat encounters are their own story vignettes

Too often people view action and story as being separate (this is an issue in books, films, and RPGs), that the two don't mix well. The opposite is in fact true. The best action scenes are those where plot development and characterization is actively ongoing. Likewise each combat should have its own mini story arc with a rise and fall in tension as it goes on. For example, the party has just arrived at an abandoned warehouse they suspect the enemies of using as safe-house. The party attacks them and they begin to lose numbers and start retreating, the party rushes forward to capitalize on this and finds themselves ambushed by reinforcements the enemies called in. Now they need to escape, they pull back into the warehouse and are followed, as they reach the other side they explode some fuel tanks and light the building on fire cutting off enemy pursuit as they disappear into the night.

It's up to the DM.

Whether or not you agree with him, your DM has the final authority in deciding which actions yield XP and which do not: (DMG 261)

You decide whether to award experience to characters for overcoming challenges outside combat. If the adventurers complete a tense negotiation with a baron, forge a trade agreement with a clan of surly dwarves, or successfully navigate the Chasm of Doom, you might decide that they deserve an XP reward.

While there are various compelling arguments on both sides of the issue, the system gives wide latitude to the DM for giving out XP.

Additionally, your argument is metagaming, depending on how you interpret the wording of The Fates. If the past was rewritten so that the animation of the corpses never actually happened, the characters might never have known that there was ever a threat there to begin with, and thus from their perspective, there was nothing to gain experience from.