Highest achievable datarate for sub-1GHz wireless
It's down to bandwidth. LoRa is wideband, CC1125 is narrowband.

The app note you link to starts, on page 6, with a restatement of the Shannon-Hartley Theorem for data transmission over noisy channels. Nobody answering this question, including myself, is going to do better than restating that capacity theorem. Read that through a few times, and understand it.

As for the actual implementation, LoRa's chirp method of spreading does offer a few neat implementation advantages over other methods, like sweeping doppler, timing and frequency accuracy all into one easy to compensate for error. But that's details, it's not fundamental.