I've not seen them mentioning 1804 combined with windows pc IP's, only about xbox alpha preview users. All pc IP's show 1803.
If i remember correctly, Xbox always has one number above. For example FCU on Xbox is 1710, not 1709.
I'm actually about 95% sure that you are wrong and that 16299.15 was rolled out as a CU and not as a full upgrade for Insiders on the Fast Ring. At least in my memory. The official blog entry from back then is a bit vague on this, but I found several tech blogs confirming just that. .15 and .19 were both cumulative updates.