Yes, it's not unexpectedly complex for a science paper. But like many academic papers, it is needlessly complex. As I've shown, you can mostly restate a technical paragraph with two plain English sentences.
I often wonder - would it be that difficult to present academic writings in simple English, or at least to provide a plain English abstract? Publishing a paper takes months; what're the extra 10 minutes to make it more effective and readable?
You've taken a paragraph that contained a lot of details and was quite precise, and replaced it with wishy washy language. For example, they day that the cells they found are heterodimers of the T1R cells on your tongue, specifically T1R2/3. You replaced this precise explanation with the single word "similar", which could mean that they are enitrely different cells that are tautologically "similar" in also detecting sweet tastes.
They were also explaining exactly what the cells detect, which you thought is not worthy of mention.
The single short clear phrase "upper GI tract" you replaced with the longer and more akward "the part of the GI tract between the mouth and the duodenum".
Overall, your phrase would sound more complex and awkward to a practitioner, and you have reduced the information significantly. There is a reason technical language is used in technical papers - it allows you to be concise, information dense, and precise.
If the audience is the layperson, then of course you are right. The ideas being expressed will be completely lost upon the audience because they won't understand half the terminology used. But I suspect you might be in the minority if you think academic journals are intended to have an audience that broad.
Of course, I appreciate the value of jargon where it is necessary to convey meaning. Although Wikipedia has shown us plain English often conveys the entire meaning and details just fine, making jargon superfluous.
This paper, however, isn't the best example of jargon abuse. Some phrasing in it triggered me and I've reacted too strongly. Overall, it is relatively well-written.
More thoughts from another simple English advocate like me - http://www.paulgraham.com/simply.html