This is common and SOP for every Disney park that has come since, too. Only now they know better and always peg the "birthday" to the date the Park first opens to the paying public. And not to the Press Preview days (where Dedications are often read in advance of the actual open-for-business date) that preceded it.
And to top off that confusion...subsequent Press Release Photos and video and TV shows...often show photos or footage of the Press Preview and yet state it is from Opening Day. A good example of this that comes to mind would be Disney-MGM Studios in Florida. Disney often shows photos or video attributed to the official May 1 opening of the park...that was actually taken on it's April 29th Gala "World Premier."
At the Gala World Premier Press Event on the 29th the ribbon was cut, the dedication given, the park was declared open...but all just for the cameras and invited VIP's. On May 1st the Park opened to cash paying guests with much smaller ceremonies. But that's when they do the "First Family" and all that. And that's the date they use for the "Opening Date."
Same happened at Disneyland in 1955. Big Press Preview Day, followed by Opening Day and "First Family" selection and all that. And for its early years the 18th was often used as the Opening Date. My theory is that darn plaque with the 17th engraved upon it is what got them to finally just ditch the 18th and go with the 17th. It got too confusing for them, so they just junked the 18th.
According to a conversation I once had with John Hench...Walt & Roy both very much considered the 18th as Opening Day. He said there was no question that was the day that mattered most to them. Roy even made sure he bought Disneyland Ticket #1 on the 18th.
The confusion continues today with Disneyland's "history!" Many 18th photos are incorrectly attributed to the 17th. Or 17th photos are attributed to the 18th! Once an error is released to the world...it tends to propagate itself, no matter how hard you try to retract or correct it. Some future researcher will find an old "17th" labeled photo that was actually taken on the 18th...and chuck that out there again, and there you go all over again. The error replicated once more.