This is a reasonable timeline and probably better thought out than I expected. Maybe I'm jaded about HP but I suspect WOSI played a large part in making this good.
There are 4 big points I took from this simple schedule:
1. This maintains the viability of the WebOS ecosystem in the short term. Releasing Enyo first in a way that lets apps be cross-compiled to other environments (cough Android) gives the devs a revenue stream from their existing apps and, since there's no need to learn a new way of coding, encourages the creation of new WebOS apps (even if the real profit comes from sales to Android users)
2. It leverages existing platforms that have quite a few developers (Qt & Linux). Using a standard linux kernel means it becomes easier to compile replacements as you don't have to add any WebOS-specific libraries. More importantly, we get to use all the open source drivers as well as any redistributable closed-source drivers that work on the platform. Qt has quite a few followers, particularly in the Moblin/Meego crowd that was orphaned by Nokia/Intel/MS, so this may convince them to port apps to WebOS.
3. Supports new hardware. Barring the release of Enyo, the release schedule appears to be working from the kernel up to the final complete OS. That means any touch-screen hardware that boots a Qt-enabled Linux can be tested for WebOS compatibility.
4. Official release is early enough (September) that if any of the smaller ODMs were planning on releasing a WebOS device that they could have it in a usable state for CES. Heck, they could probably have hardware ready for Christmas. (Assuming final validation/tweaks to the software September while the pre-qualified hardware is being assembled, finalized WebOS flashed & shipped from Asia in October, starts shipping to users mid/late November)