Google has backtracked a bit on this, they released this statement earlier today
[Google has circled back with us for some clarification. While only the Pixel Stand will charge the Pixel 3 wirelessly at full speed right now, a certification process is underway through the company's Made for Google program to allow third-party chargers to operate in the same manner, at full 10W speeds.
These certified chargers will take advantage of the protocol currently used by the Pixel Stand to similarly charge the Pixel at 10W. Belkin's got one in the works, and presumably we'll see other accessory manufacturers follow suit.
While the third-party chargers will be formally certified and able to operate with the Pixel 3 at full speed, Google's unique charging UI will remain exclusive to the Pixel Stand.]