There's enough evidence to make an educated deduction.
1) D&D has traditionally released at GenCon with 3.0, 3.5, 4e, and Essentials premiering then (or immediately prior).
2) WotC did not confirm"release date" leaked by B&N.
3) WotC did not announcing a release date at Winter Fantasy, GAMA, or Pax East
4) The GenCon crunch for publishing companies is the first couple weeks of April. According to the Paizo editor-in-chief, for them it was the 14th to 18th this year, when they *had* to have books finished to be ready for the Convention.
5) At Pax East, April 10-13, Perkins said D&D5 was 97% done.
This strongly implies WotC was not certain the game would be ready for the desired release of GenCon, prior to this week. If anything delayed the book by even a couple days, such as a last minute rules error, the book wouldn't be out for the Con.
Which meant they were cutting it *really* close.
It's highly unlikely WotC would want to wait until the very last minute to have the game finished, so this implies delays. Repeated delays.
At this point, WotC knows for certain if they will make GenCon or not.
If it is GenCon, they can release that information now. If they can't make GenCon, they'll know it will be out shortly after, since they would have *just* missed the deadline. Likely September as they release at the end of the month. So it's safe to release that later date.
Which is also why they were suddenly hyping Tyranny of Dragons. They weren't certain D&D5 would make the cut-off date but had to do something at pre-scheduled panels and something to get gamers excited for their GenCon offerings.
Even assuming, as others have pointed out, that the Starter Set isn't what was intended for a GenCon release or Summer release, I still think you're assumptions are wrong.
You have a major flaw in your theory. Deadlines for China printing are not nearly as fixed and tight as you think. They are highly variable, and depend on factors such as 1) how often you print with that company, 2) the quantity you are printing (more can actually be better, which runs contrary to most people's assumptions), 3) your shipping agent relationship, 4) your pre-paid customs situation, 5) which factory is manufacturing it, 6) how close you come to Chinese new year, 7) whether you own or lease the factory or a fixed portion of it, 8) whether you have sufficient quantity to fill a container, or to fill an entire vessel, or have ownership or lease rights to a vessel, etc..
Whatever the Paizo employee said, he wasn't likely giving you an actual hard date for what was possible, just the date they had worked out as the most "safe" date, with some room for things to go wrong (I know from experience a shipment can be held up for 3 weeks or more by an unexpected customs inspection by customs agents, or a longshoreman slow-down). And, he was not saying it was universal for all print jobs for all companies, just for their company.
But Hasbro is MUCH MUCH MUCH bigger that Paizo when it comes to printing. I think people forget just how much printed material goes into Hasbro productions. It's likely they own or lease the factory, or otherwise have an exclusive relationship with at least a portion of it, in a way that Paizo simply does not. The time frames for a factory you control are drastically different than those for one you're simply using as a third party. Similarly, if you fill an entire shipment with materials, shipping changes drastically as well. Even just filling a container changes things.
My point is, your assumptions on how long it takes to print something in China for Hasbro, based on how long it takes for Paizo to print things in China, is seriously flawed.
And once that assumption goes, all the rest collapse as they're all dependent on that assumption. If they could get the book printed and have the shipment arrive in a single month for example, then all the rest of your assumptions can be simply chalked up to "not wanting to give out information until they are closer to the date of release". Which is what I implied in my initial reply - you want more information, and you argue it damages them to not give out more information, but others disagree with you on both those points.
