Microsoft was already set to spent 27 billion usd on research for 2023. They dedicate huge standout double digit percentages of budget to research every year. Their in house AI research division was already huge.
They didn't become a major AI player overnight... They already were long ago.
OpenAI is small, in raw numbers of AI researchers, compared to the big players in the space. That's a major reason why it's so compelling that they have been able to consistently set the bar for state of the art.
They were a dream team... But small. Msft is adding AAA+ talent to their existing A+ deck. Also they won't have to rewrite the code base. Can hit the ground running.
Lastly, there is no evidence that openai has the greatly quoted and so hard to define 'agi'. That's Twitter hearsay and highly unlikely... If folkes can even agree what that is. By the overwhelming percentage of definitions... Even gpt-5 is unlikely to meet that bar. Highly speculative. Twitter is a cesspool of conspiracy theory... Don't believe everything you read.