He is saying that what might seem like a sophisticated, well-planned strategy (akin to playing a highly complex game like 5D chess) could actually be just the outcome of basic errors or poor decisions made by someone else (similar to making simple mistakes in an easy game like checkers).

"Never attribute to malice that which is adequately explained by stupidity"

In this case, it means that what happened is: “OpenAI board is incompetent”, instead of “Microsoft planned this to take over the company.”

A conspiracy like the one proposed would basically be impossible to coordinate yet keep secret, especially considering the board members might loose their seats and their own market value.

I think it means don't attribute to intelligence what could be easily explained as stupidity?

OpenAI may just be a couple having an angry fight, and M$ is just the neighbor with cash happy to buy all the stuff the angry wife is throwing out for pennies on the dollar.

In other words - it doesn’t have to be someone’s genius plan, it could have just been an unintelligent mistake

Hanlon's razor, basically.

The most plausible scenario here is that the board is comprised of people lacking in foresight who did something stupid. A lot of people are generating a 5D chess plot orchestrated by Microsoft in their heads.