Companies can support more then one OS. You know, Sammy has Android, Windows Phone, it's own OS and soon to be Tizen. HTC has Windows Phone and Android. LG has Windows Phone and Android. You don't have to be all in, since people buying Windows Tablets are going to be different then people buying WebOS tablets.
What this tells me isn't that HP is waffling. It isn't WebOS had no offers on the table. It tells me HP spent months deciding if Android was the right "second OS" that Meg wanted, and was quoted saying before this announcement, that "HP needs another OS" (source). HP surely looked at the state of Android, the fragmentation and looked at the numbers -- saw that TouchPads sold more then any one Android tablet this year -- and said WebOS has a better chance of being that second OS. So Windows 8 might be the focus, but it wouldn't be hard to make an awesome Windows 8 tablet and port WebOS to it in 2013.
And remember, the people who designed the TouchPad are gone. So I would bet that the people who will design the HP WebOS tablets will be the same who is designing Windows 8 tablets.