Based on what?
Not the OP, but it’s far easier for a US military or civilian agency to be conducting drone tests in the vicinity of the US than anyone else. Plus the US govt has a history of warning about adversaries wielding covert weapons while they are already using them, e.g. Stuxnet was already destroying Iranian centrifuges while Obama was warning of cyber warfare.
One possibility is that this is intended to reveal to the Chinese that the US has drones capable of intercepting naval vessels, in order to dampen adventurism in the South China Sea.
By doing it this way, they avoid the risk of a drone being captured and the capability being properly evaluated. This would create maximum uncertainty. It also reduces the justification for posturing and overt escalation.