Gotcha. I appreciate you walking me through your thought process. As a student of a few martial arts, I know exactly what you’re saying.
When I read the Kobold Press marilith, it sounds like it could Parry multiple attacks from one player in the same turn. There may be a universal rule “one reaction per turn” preventing that…but the wording change from the 5e marilith (where it’s abundantly clear - “Reactive. The marilith can take one reaction on every turn in a combat”) creates a hiccup for the GM.
Why introduce vaguer language just in order to limit the marilith to using Parry only, and not Opportunity Attack? Because if it can Parry all 3 of the fighter’s attacks, that’s a hard down beat & prolongs the battle (which 5e doesn’t need except in the case of solo monsters - which the marilith as a commander is not).
So if we’re saying “it’s because the marilith is a tactician blending offense and defense”, there’s a more elegant mechanical implementation - Riposte, where if her 1/turn Parry succeeds, she can make a counterattack. This speeds up combat, emphasizes the “defense as offense” that you cited, and it can be done without vague language / limiting to Parry only.