No they weren't patched the ENTIRE DLC's were reuploaded to fix it you had to delete your DLC data and redownload them.
They fix the bugs then reupload a new copy of Dawnguard for download thats how the beta worked. The patch in question was in preperation for Shivering Isles it all fixed stuff that effected the main game it didn't do anything for Shivering Isles its self. Try reading the patch details.
I did read the patch details, and there are two versions of that particular patch - one for those who DO have Shivering Isles, and one for those who don't. Both are fairly small files. As such, Shivering Isles has its own patch.
That being said, DLCs are patched (meaning: patch, not download a revised DLC) all the time for other games - it's not new. Games like Call of Duty have them frequently enough to fix glitches on their maps.
EDIT: Good grief, I'm not going to get into an argument about this, so I'll just clarify what I mean here.
You are confusing if a DLC *can* be patched versus if a DLC *won't* be patched.
I was simply stating that it can. Patches (from an Xbox-context) are essentially binary offsets that load (generally prior to the game itself). What the main game is DOESN'T matter. This can be on disc (which is the reason for offset code, rather than modified code), on "Games on Demand" format, or DLC. This is how it is.
Now, most XBox DLC (regardless of game), is generally small. So it doesn't really make sense to have a binary offset to patch this, so the DLC itself is completely replaced. This is where the previous discussion of the smaller Oblivion DLC being replaced by newer versions comes in.
However, not all companies will patch DLCs (as a side note, consider costs of patching on the XBox 360 platform). Bethesda is a company that seems to work on one project at a time. They are not likely to issue a patch for a previous game/DLC when a new one has been released (they'll focus on the new one). The fact that you believe patching DLCs isn't possible shows that their PR spin is working.
Okay, with that said (yes, DLC patching is possible but will it be patched?), comes the question of Dawnguard. Will it be patched or won't it be? Your guess is as good as mine; however, I believe it will due to: 1) obviously the Bethesda/Microsoft deal brought some flexibility to the table (e.g., the previous enhancement patches, etc.), 2) the fact there was actually a beta test with feedback, and 3) there are some glaringly obvious bugs in Dawnguard so far. I guess we'll have to wait and see.