It's not /dev/urandom written to disk, no. Yes, a raw format has a structure. There's not one "RAW" format though (and TFA notes this): e.g., my Canon's RAW format specifically referred to is called "CR3". And its predecessor was "CR2", so even within a manufacturer there are multiple such formats. All undocumented.
But a Pentax won't write out CR3s, it'll write out some other, yet bespoke format.
I've vaguely reverse engineered some of CR3: it is a container that contains multiple copies of the photo taken; IIRC it contains a thumbnail JPEG, the JPEG, and the raw data itself.
I doubt it's the most performant to write to storage: the format is vaguely TLV (its fairly similar to RIFF, if you understand the RIFF format), so it can't really be streamed to storage due to needing to know the lengths of the containing chunks (all the way out to the outermost chunk).