The fact that these people conflated "grammar" and "inflection affixes" makes me think they don't know what they are doing.
> "Many of the disagreements are down to differences in how 'complexity' is defined [...] In this study, we improved the methodology by teasing out two distinct measures: fusion (how many affixes verbs and nouns have) and informativity (how many distinctions are made)."
It strikes me as more likely that they're focusing on easily quantifiable metrics of grammatical complexity. Without reading the paper myself, I'd guess that the metrics chosen here suffer from lamplight fallacy (they're focusing on what can be quantified because it can be quantified as opposed to working out what would be a useful guide).
I just started learning Mandarin this year. It's somewhat famous for having a simple grammar, but that in no way means that it has lesser expressive capacity. It just means that the language has evolved different ways of expressing the same ideas. They don't happen to be ways that can easily be summarized as grammatical rules. But I'm not sure I'd take that to mean that it's a simpler language. I think it probably says more about the concept of grammar as defined by grammarians.
(Worse, Chinese language instructors say "grammar" but mean "inflection", which makes Chinese grammar textbooks an infuriating affair.)
Usually that's fine because most readers don't actually understand what they're reading.
And then you turn the page and again assume they know their shit.