I think, ironically, emotional absolutes is a phrase using overly emotional language of the type the reviewer would like you to avoid. It may have been more impartial to have phrased it something like the authors should avoid emotional language or stated positively as the authors should use more objective language. You may find this rewriting to be more concrete because you aren't left puzzling over what exactly is meant by emotional absolutes - it just sounds like a bad thing, accusing the writer of absolutism.
Emotional or emotive language is language directed towards the emotions of the reader, often in a way that tries to persuade the reader to agree with the writer by appealing to values and feelings.
It's common to advise that academic/scientific professional writing avoid emotive language because it conveys subjective interpretations that are unlikely to be supportable by empirical evidence. That said, this is somewhat of an issue of style, and writing completely impersonally can come off as too dry.
When you describe findings as either inspiring or discouraging you are really describing your personal, emotional response to those findings. Findings can be significant or inconclusive without introducing emotion. For example, it might be disappointing to you if your technique is not shown to be conclusively better than an existing technique, but is it really objectively disappointing? For all the people who have used the technique in the past it might instead be reassuring or a bit of a relief. You can simply describe the result as inconclusive and allow yourself and your reader to have their own personal response.
Since I don't feel like it's possible to give a concrete definition that covers all cases, I can share some other examples from style guides and academic writing advice that cover this issue:
To find language in your writing that is overly emotional, I would suggest going through, sentence-by-sentence, and ask yourself whether the sentence is something that everyone could agree on or whether it's really your subjective view.
As a final piece of advice towards interpreting reviewer comments, I'd also suggest that
the authors deal in emotional absolutes
may be code for:
the authors express opinions that I disagree with
The reviewer may be most placated if you defer to the existence of controversy on some of the opinions you express. You may be writing that there are widespread pessimistic views of their favorite technique, and while they may not expect you to delve into all the pros and cons of both sides in your particular manuscript, you can at least share your opinion without implying that those who disagree are idiots. In statistics, there is a common phrase "All models are wrong, but some are useful", written in various forms by George Box and some others. I think this phrase is also true when expanded as "All techniques are wrong, but some are useful" - keep this in mind when writing a paper that is critical of various methodologies.