That's just your speculation and not supported by the facts. Why are they dumping so much money into free HTML5 dev tools and support for HTML5 apps in Webkit if their goal is to prevent cross platform apps?
Apple's move to support HTML5 is commendable, but don't doubt for an instant that they also have an interest in breaking the back of Flash. Adobe has been less than stellar in supporting the Mac lately and Apple wants to be sure they aren't beholden to them in the future.
Also, why would taking steps that make cross platform development in and of itself help Apple instead of hurt them. Generally, breaking interoperability only helps when you have dominant market share, otherwise it hurts the bottom line. So basically, your hypothesis has no support.
Apple's mindshare with casual users is tremendous. The big fight right now is not among Apple, Android, RIM, Nokia et al, but rather between Apple and Android. The latter two have traditionally targeted the business and prosumer markets and have mature offerings there. Android and the iPhone OS are both new, emerging platforms targeting home users, and in that market Apple's position is enviable. They may recently have slipped behind Android on total sales, but they still have the next gen iPhone coming down the pipe and all reports indicate that it will close the hardware gap that emerged with the release of the Nexus One.
Your argument that Apple's exclusion of Flash is based exclusively on concern about application quality is a little naive, I think. If applications developed with Adobe's tools are lower in quality than the native offerings, consumers will recognize that and opt not to use them. Saying that they will instead turn around and blame Apple for their shortcomings is a little twisted. Additional choices are a win for consumers here, the only one who stands to lose is Apple. Denying that their actions here are not motivated at least in part by unadulterated self-interest is disingenuous.