Still seems fairly speculative to me. I think it's very likely that there was life given all the water but this is still a ways away from a smoking gun.
You are right: this is indeed no smoking gun (and it isn't hyped to be one). This is more like "we can't rule out life having created this, but there are alternate explanations which have also not been ruled out".
Unfortunately most of the evidence is going to be like this. The chances for better evidence would probably require a sample return of some sort, and even then I wouldn't expect a smoking gun (either way).
Early on I would expect a whole lot of "horizontal gene transfer" sort of things to have taken place. So for example in addition to actual horizontal gene transfer, there are mechanisms like one organism enveloping another to eventually become organelles, co-opting products from each other, etc. All of which would act to homogenize life and make certain process ubiquitous.
Finally, there's an outside chance that "there's only one way to do it".