I fully agree, Denim *should* be the Lumia specific firmware. But the confusion is amplified by Microsoft. Why? Because Denim, the real thing, was rolled out only on some 930 and 1520 (Nordic countries). But in the same time, they rolled out GDR1 on older devices (just like they had sent GDR1 on the 930 in October). These "OS only" updates come with a minor Cyan update. But in the update table issued by Microsoft, they're tagged as "Denim". And, what is more, Windows Central is not very precise in the different articles, calling Denim whether the firmware, the update operation, the software updates, etc.
So people read and say "I have Denim on my 520", when they've just received GDR1 with a very minor update to Cyan. Even Microsoft confirm it, as they qualified the 3058.50009.1447.0001 firmware being tested on Finnish 1020 and 925 (versus 3058.50009.1424.xxxx for all other 1020/925) as a "Cyan update ahead of Denim roll out".
Microsoft sending out only GDR1 on older devices (except the Pureview ones, 920 925 and 1020) doesn't exclude a Denim firmware later.
But to disguise the delay (with the help of the extras+infos bug), Microsoft have started to call any update Denim.
The HTC 8X should receive GDR1, which is not Denim, even if it's being called Denim right now by Microsoft in their own Lumia update table, and amplified by approximative articles on most sites including this one (with all due respect).
In other terms, if the OP concern is: will I get all the Denim goodness (ie everything but the camera things), the answer is: you'll get these when you have GDR1 (an OS update, not a firmware one).