This “discovery” was stumbled upon by Ron Amadeo, who went through the commits to the Android Open Source Project (AOSP) and saw the relevant evidence. A commit that hailed from late August was labeled as “shamu: remove fingerprint support”, which pointed to how prototypes did make use of a fingerprint scanner from Validity Sensors, Inc, which is a company that was picked up by Synaptics in the previous year. Apart from that, the company has also pointed to clear touchscreen-based fingerprint scanners before.
Of course, there were also commits to AOSP that actually hid the fingerprint API code in Lollipop, at least until the new API is ready. It is said that this would be a “swipe style” fingerprint scanner that is found in Samsung devices, as opposed to Apple’s TouchID sensor that sees action on an iPhone at the moment.