You have a source for this statement?
Internal discussions within the semiconductor manufacturing industry have pegged the approximate original Bill of Materials for the Steam Deck at $450.
It makes sense for a company like Valve to manufacture a loss-leading product. because Steam Deck exists primarily as a marketing tool for Steam. Let's say the average enthusiast who buys the Steam Deck also purchases an extra $300 worth of games to play on the Deck throughout the Deck's lifecycle. At a 30% cut off topline revenue, then 64 GB ARPU would be $490, breaking even for Valve.
So even though they would initially lose money with each 64 GB SKU sold, they would recoup enough to justify it through Steam purchases, and at the same time promote engagement with the Steam ecosystem. Given that Valve has persistent Epic Game Store and Xbox Game Pass competition in the PC market, Steam Deck provides needed value proposition to justify its brand and keep consumers loyal to Steam.
However, this has definitely declined since late 2021 when the original Steam Decks were manufactured due to economies of scale. And, I can guarantee you that the $529 and $649 SKUs weren't loss-leading as Valve followed the Apple playbook and charged an extra $130-$240 for a negligible bump in costs. But there is merit to the claim that the $399 64 GB SKU was loss-leading at launch.