It seems that Valve had incorrect details on the specification sheet for the Steam Deck, and as a result we now know the RAM is more impressive.
Originally (as seen on the Web Archive), Valve listed the Steam Deck as having "5500 MT/s dual-channel" which they've now adjusted to say it's actually "5500 MT/s quad 32-bit channels". Thanks to that, we know that the Steam Deck should perform even better than we originally thought it would. The speed may not have changed, but dual to quad is still a pretty nice boost.
We've known for some time now that RAM speed and bandwidth is important to Ryzen CPUs, so this is quite an important clarification to come from Valve. For such a small (relatively speaking) device, every little bit of possible performance is going to be vital, especially since likely the majority of games will end up being played using the Steam Play Proton compatibility tool.
Additionally, Valve designer Lawrence Yang has clarified that when the Steam Deck was recently shown off to IGN the games were actually being played from an SD Card:
Yep, games will load faster off internal storage, but games still play great off an SD card. When IGN came by, all the games they tried (and shot footage of) were played off a microSD card.
Yang also confirmed on Twitter that when docked you can run Steam Deck games off of an external SSD.