The paper does not seem to support that human speech has a universal transmission rate or that every spoken language has a universal information rate. They showed that information rate varied by individual and by language, just that it varied less than the syllable rate.
This was also bounded by a reading task, so the performance shown per language could be influenced by the average reading skills of people who speak those languages. They also asked them to pronounce differently than they normally would.
If you take the 17 languages they tested and get the average between them, you get 39bits/s. For English and French, the information rate they recorded was higher, with an average closer to ~45bits/s (just eyeballing their chart). Their results also showed Thai at ~35bits/s. A 10bits/s swing from median to median is pretty huge.
From the paper:
"We collected recordings of 170 native adult speakers of the aforementioned 17 languages, each reading at their normal rate a standardized set of 15 semantically similar texts across the languages (for a total amount of approximately 240,000 syllables). Speakers became familiar with the texts, by reading them several times before being recorded, so that they understand the described situation and minimize reading errors"
"Together, our findings show that while there is wide interspeaker variation in speech and IRs (information rates), this variation is also structured by language. This means that an individual’s speech behavior is not entirely due to individual characteristics but is further constrained by the language being spoken."
"However, languages seem to stably inhabit an optimal range of IRs, away from the extremes that can still be available to individual speakers. Languages achieve this balance through a trade-off between ID (information density) and SR (syllable rate), resulting in a narrower distribution of IRs compared to SRs. In the introduction, we rhetorically asked whether too low or too high an IR would impede communicative and/or cognitive efficiency. Our results here suggest that the answer to both questions is positive and that human communication seems to avoid two extreme sociolinguistic profiles: on the one hand, high ID languages spoken fast by their speakers (“high-fast”), and, on the other, low ID languages spoken slowly by their speakers (“low-slow”)."