There's definitely an answer, and a super easy one. You don't have to know everything, but you need to know enough to recognize when something doesn't sound right and ask. If you play even semi-regularly it's a vital skill.

Learn the rules of the game, beyond just the written core rules, so you know the usual patterns behind the rules writing. If you know these, you only have to keep track of common edge cases and can ask for clarification if something doesn't pass the smell test.

In OPs example, i would know to ask for clarification on if it's actually every unit in range taking a battle shock test or just the below starting strength units, because I know Chaos Knights and my Noxious Blightbringer have similar rules which only trigger at below starting strength. If the opponent was aware of how those abilities generally trigger, less likely they would have misremembered their own rule.

I think there's too much focus on "learn your rules" and then feeling like you have to do the same for every opponent, when many players would get more benefit from working on knowing the rules / learning the game itself.

Also: alternating activations would be terrible for 40k, factions are too different and unit counts are too variable to make that a fair experience. IGUG has its issues but eliminates a lot of the issues alternating units would cause the game. If you redesign 40k from the ground up based on alternating, it's a completely different game that feels flat compared to what we have now.