Sony going on radio silence for PSX and E3 was a surprise, but an interesting move. My gut tells me they're trying not to spread out conferences with little content and just give a well-packed conference at the right moment when they're not overshadowed by six conferences before them.
The statement does say "We are exploring new and familiar ways to engage our community in 2019 and can’t wait to share our plans with you." so it wouldn't be too crazy to expect one or two independent conferences chuck full of next-gen announcements like a PS5 hardware and pricing reveal and new/upcoming software. Who knows? Maybe they'll reveal some "Exclusive" and "World Premiere" first and third party games before E3. I'm sure they just want to see how hot the new technologies will be in the future and not make a mistake. Like going fully digital/streaming and getting completely shunned by customers and critics, or fully stick to physical while other companies make a full jump into the future, leaving Sony behind unable to catch up on both instances.
I guess they just want Microsoft, Nintendo, EA, Ubisoft and the rest to lay their cards on the table first. This is a chance for Sony to work in "secrecy" on whatever they'll do for next gen without someone getting the jump on their projects.