All they said for sure was they sold out their first two production runs which were based on pre-order numbers, and that was over 1mil at the time they said that. Found this quote though:

"According to Omdia, the Steam Deck sold an estimated 1.62 million units in 2022, and is on track to sell about 1.85 million units throughout 2023. This would push total Steam Deck unit sales to 3.47 million by the end of 2023."

So if true, they blew through those first units super fast, and then ramped production again. I'm sure they sold a ton last month when t was 20% off for Summer Sale as well.