Because of past history. Patents don't float around. This was not a leak. Every single patent gets published and this is not something new, even for Disney. This would be a big exception to the pattern. Disney files patents for ride systems and doesn't intend to build them. This is nothing new. This is completely normal. Disney filing for a patent and it being part of an unannounced, impending project would be new, would be abnormal.
As an example, the patent related to Mater's Junkyard Jamboree was not filed until 2008. That would be after the Cars Land announcement in 2007.
I think this is where you are incorrect about this situation. While vague during D23 Chapek did announce Spider-Man and Avengers would be the two attractions that are part of Marvel Land. So this is not an unannounced project being discussed.
Yes this particular patent being discussed could be part of something else or even nothing at all. However its not that big of a stretch for it to be used for a Spider-Man themed attraction, especially since Spider-Man is directly referenced in the patent.