Here's the quote from the presentation that refers to the extended development period and the future announcement of the release date.
...the upcoming release of Shenmue III has been moved, and will now release in 2019. The extra time will be used to polish the quality of the game even further to the high standards it deserves, and release the product in the best possible time frame. For now we will say 'coming in 2019' and will update you at a later stage with an exact release date. And we hope that when players get hands on it, they will ultimately think the extra time was well spent.
The two parts I bolded are ones that jumped out at me. First, setting aside from the matter of "polishing" the game:
"The extra time will be used to ... release the produce in the best possible time frame." So in other words, aside from continuing to develop/improve the game, the extended development time is at least partially due to a desire to position the release date strategically. What this means is completely open to interpretation. Personally I would think they might try to avoid the annual September-December flood of AAA titles... however, they might have their own market research that says that this would be the best time for Shenmue III, so it's anyone's guess.
The second phrase says that they
"will update you at a later stage with an exact release date." Unless he said anything more specific elsewhere in the presentation, this could also mean anything. Will the announcement be at the next public event? Five public events from now? It's completely open-ended.