It might imply a patent license, it might not. I don't think it's ever been tested in court. But at best you're getting a license to the Preact author's patents, if there are any, which there probably aren't. You're definitely not getting a license to Facebook's patents - but because Preact is a clone of React (in outward design, at least), any Facebook patent that applies to React has a decent chance of also applying to Preact. So I'm not sure it helps that much...
Apologies. I didn't notice this was about preact specifically. I agree that switching to preact is unlikely to help with any patent issues. I don't think any free licenses indemnify you from third-party patent claims.
