When something becomes popular, people often look for people who "predicted" it. I remember the article about how Microsoft's Nathan Myhrvold "invented" the iPhone in 1991. Except that he didn't. Microsoft made a hand-held computing OS before the iPhone and Palm Pilots existed a decade before the iPhone. In broad-strokes, they're the same thing. However, they weren't popular (and not for lack of trying). The issue wasn't the broad-strokes, but the minute details that made the iPhone (and other devices since) a joy to use. His work probably led to Windows CE, PocketPC, and Windows Mobile. He didn't invent the iPhone.
Similarly, before Dropbox we had options like XDrive from AOL, WebDAV servers, FTP, iDrive from Apple, and many others. We were awash with crappy software to store our documents on a server and access them from any computer we were on. Even Microsoft offered a slew of sub-par options that would get re-done as a Dropbox competitor as everyone started recognizing that Dropbox's model was wonderful.
Giving David Gelernter credit for predicting Dropbox is to show a lack of appreciation for the decisions that made Dropbox great - and better than the army of options that came before it that simply weren't good. Dropbox did what many before it couldn't do: create an online storage system that people love. I remember trying a bunch of services like XDrive back in the day. Transfers would fail, the transfers were synchronous (so you'd get a pause until the write had happened), things weren't cached locally so load times were poor, the mount was finicky, etc. They all satisfied the "making documents available everywhere" criteria similar to Dropbox, but they simply weren't good. Dropbox worked hard to make it good. They made lots of little design/technical decisions that make it Just Work. Many before them had tried and failed.