I think the main issue was he used Nintendo owned tools and libraries to make his game instead of the GPL ones, making the release of the port dependent on Nintendo's approval too. I guess even Valve didn't want to deal with their lawyers.

In principle he could use alternative tools, like libdragon, but he said even if he did that it was unlikely Valve would permit it, as Nintendo would still be antagonized somehow. And Valve it seems wants to improve their relationship with Nintendo (See: Valve blocked Dolphin on steam, and took down a video showing yuzu installed on the steam deck).