and
"no longer has confidence"
points to something more serious than underestimating costs.
"not significantly candid in projections for profitability"
"not significantly candid in calculating operation cost increases"
"not significantly candid in how much subscribers are actually using ChatGPT"
etc.
Maybe Sam had been trying to broker a sale of the company without consulting the board first? All speculation until more details are revealed but he must've done something of similar magnitude.
Of course we have no clue if that's what actually happened. Any conclusions made at this point are complete speculation, and we can't make any conclusions more specific then "this is probably bad news."
To be clear, this is only one possible explanation for Altman's firing. And for my money, I don't even think it's the most likely explanation. But right now, those who rely on OpenAI products should prepare for the worst, and this is one of the most existentially threatening possibilities.