Not to mention that "speculation", better known as theory, is a fundamental and crucial part of science. Some of the most celebrated scientists in history are celebrated for their theoretical work. Newton, Einstein, Charles Darwin(sure, Darwin did a lot of observation as did most biologists of that era, but his theory of natural selection, though inspired by his observational work, is clearly a theoretical idea, not an empirical result).

And of course, skimming over the actual paper in question, it's not even theory/speculation really. It's more like a review of the existing literature and empirical data on soda lakes, the different types of them, and their plausibility for abiogenesis based on their ability to maintain P concentrations despite significant biological activity.

This is actually someone testing a previously theorised idea against the best available data and affirming it as plausible.