The thing is every PC is a "gaming PC". The only distinction is how powerful you make it. And PC manufacturers generally sell a line of laptops/desktops branded for gaming. I found random numbers from 2021, where Lenovo said that they sell over a million of their Legion gaming laptops every year (https://news.lenovo.com/pressroom/press-releases/lenovo-legi...). Has Steam Deck sold more than a few million since it launched in 2022? Unlikely.
A gaming PC is a PC that's marketed as a gaming PC. Lenovo obviously isn't marketing their Thinkpads as a device for gamers, even if they'd be happy to sell them one.
Legion isn't a single device. How many models under the Legion branding are there? Even if they sold a million and change, break that up amongst multiple formfactors and I'm sure the Steam Deck outsells any individual one of those devices.
