It is the night before our last session to finish Tomb of Annihilation.
For the previous three sessions we had seen things in the tomb that gave us a clue to what creature we might face. [spoilers]
...Our party saw eye stalks popping out of purple mold all over the Tomb. The party saw a door with 10 eyeball "keys." The party suspected for weeks that they were probably going to face a Beholder - just as described in Volo's Guide being sold in the Port of Nyanzaru. One player had once used Fog Cloud to shut down a Beholder in a different AL adventure just a few months earlier. He had cast it wrong so it didn't cover the whole area and the party almost died.
The player decided the next time he used it he would try to cast it right. This time, after it seemed clear what the party would face, the player decided he would again try the strategy of using Fog Cloud.
To prevent any rules confusion, the player posted this question on RPG StackExchange [spoiler link] so there would be a link to share if any debate or confusion came up.
The group had previously found RPG SE to be useful to quickly solve many rules confusions though occasionally looking things up slowed the game. For example, the party got confused about how a Fog Cloud canceled advantage and disadvantage. The player used a link from RPG SE to solve the confusion.
Then the adventure started. Sure enough the creature mentioned above was encountered. The player cast the Fog Cloud but it was dispelled for unclear reasons. The player thought perhaps an unusual object in the room was the cause but wondered if the DM might have had a different interpretation about how a particular skill of the creature interacted with that particular spell.
After the unusual object was destroyed, the player cast Fog Cloud again. This time, he said, "Now that object is gone - maybe this will work this time and it seems like it should work according to this" and he attached the link to the RPG SE question. [spoiler]
The DM asked, "did you just look that strategy up?" The player said no - it was an idea he had already had.
Later that night, the DM said that what that player who cast Fog Cloud did was "serious" metagaming equivalent to cheating and posted a link to wikipedia on the definition of metagaming.
To clarify, the Fog Cloud was an original idea of the player - not something found on the web. The player was accused of "cheating" for using their own idea in another game and for referring to a post they themselves created that showed their idea to prevent any rules confusion.
Per the DM, posting that rules clarification was cheating. We want to know if there is any support in AL for that interpretation.
The question is:
What guidance or criteria for AL exists for determining whether this is "serious" metagaming, and equivalent to cheating, for this specific situation of when a player uses the same strategy they used before on the same creature - but before using it posts the strategy on RPG.SE to ensure that there isn't a rules debate?
As this was AL play, the player could have argued his PCs knew each other. By a strict definition of metagaming, almost any use of a players brain using previous experience could qualify as metagaming. This question isn't focused on that binary, fine grain determination of "is" or "isn't" but rather how do we determine whether this is a "serious" metagaming that qualifies as cheating, violating the player-DM tenets of the game, and unacceptable in AL play.