I think you are placing a ton of emphasis on this, but there are many mitigating factors in a game of codex:
-Haste
-Patrol Zone Bonuses
-Combat Tricks
-Removal
Yes, P1 has an advantage and can make the initial good trades, if there are any. The original post was outlining these advantages and disadvantages. Sometimes all your opponent lets you do is damage their base early. Much like Magic, managing your life total as a resource is key. All the base damage doesn’t matter if they can’t finish the job. I still think that a conversation about P1 and P2 roles is more important for Codex than Magic because you start with so much more gold (mana). In a game of Magic, going first lends itself a big advantage especially when 2 decks are racing. Going first means you essentially are 1 turn faster than your opponent, which is a huge deal.
However in Codex, while you do get to set up initial trades and will have that 1 extra virtual turn, the benefit is smaller because P2 can set up a lot of blockers, always, with any starter deck. In Magic you can stumble on lands, which is an advantage to P1 if they are aggressive. You can also just not draw your cheaper cards, which again is an advantage to an aggressive P1. These situations rarely happen in Codex. While you may not draw your best starting unit in your first 5 cards, you will definitely have access to it in your next 5 cards (barring Thieving Imp and variance). This is a huge boon to defensive play! Aggressive magic decks are designed to be consistent and low to the ground, filled with cheap creatures and cheap spells so that stumbling on mana is minimized and not drawing cheap threats is rare. However slower decks can have those bad luck moments of drawing all their expensive finishers first and their roadblock creatures too late. In Codex this scenario simply does not exist.
Consider a mirror match a Mono-Red vs Mono-Red - how much do you base your decisions on whether you are P1 or P2? It’s a big deal for when you summon your hero, how you assign the Patrol Zone, and what you tech. The decks are theoretically the same, but your decisions will change based on whether your are P1 or P2.