It's not via mail, it's via discord; there are more details in the submission:

"I set up a channel on a reasonably-popular RPG discord server I’m on, then each commander gets their own thread, using discord’s thread feature. Each commander then gets a little doc with their character writeup and a sheet with their army numbers. They write messages to me, I reply and notify them as events occur, and I keep track of everything on a big spreadsheet and a running Photoshop map file. (...)"

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Also note that in early wargames writing down orders was part of the game, e.g.:

"Players do not speak to each other. Instead, they communicate with their teammates and the umpire through written messages. This is so that the enemy team cannot hear their plans. This is also so that the umpire can delay or block messages if he feels the circumstances on the battlefield warrant it. In the early 19th century, officers in the field communicated over long distances through messengers. There was no radio in those days. Messengers needed time to reach the recipient, and could be delayed or intercepted by the enemy. The umpire can simulate this problem by holding on to a player's message for a round or two before giving it to the recipient, never giving it, or even give it to the enemy."

https://en.wikipedia.org/wiki/Kriegsspiel