I think the real reason was that whoever you are talking about did not believe the claim, not that they found such a claim insignificant if true.
You may be right about that. Most scientists probably don't believe the 90 day claim, or any of the hundreds of smaller claims, such as 5 W for a week. They don't believe anything.
On the other hand --
I have encountered a surprising number of scientists who do not understand the difference between power and energy, such as Morrison (https://www.lenr-canr.org/acrobat/Fleischmanlettersfroa.pdf, p. 6). If you do not understand that difference, you will not understand why a 90-day, 100 W reaction is anomalous. Other scientists do not understand the significance of it. Some, such as Ascoli, make up absurd reasons why that might be a chemical reaction. Plus there are people like you, who do not understand the difference between input power and noise. You think that high input power degrades the signal and makes the results less convincing, or perhaps less believable or significant.
People who make such elementary mistakes are incapable of understanding the science. They cannot be convinced. It would be like trying to convince me that string theory is correct. I have no clue what string theory is about. I have no knowledge of that level of subatomic physics. I cannot hazard a guess as to whether string theory is right. People who do not understand the basics of cold fusion -- such as the difference between input power and noise -- cannot accept the claims.
If it really is that noted authorities discounted well documented, irrefutable claims of continuous 100W excess power for 90 days, on the basis of insignificance, please cite it.
There are few documented examples other than Morrison and Kreysa above, because most the scientists who reject these claims know nothing about them. They have never read the papers. They just say they are sure the experiments are all mistakes, fraud or lunacy. They never talk about specifics because they do not know any specifics. For example, if you were to ask them "what error did you find in the reflux boiling calorimetry that produced a 90-day, 100 W reaction?" they would have no idea what you are talking about. In such cases they often tell me "you made that up." I point to the paper, and they do not respond. It isn't as if we are having an academic debate. They are making ignorant, unfounded assertions about something they know nothing about. They are not writing papers showing errors in the experiments. As far as I know, no skeptic has ever found an error in any paper. (Morrison and Shanahan think they did, but anyone can see they are wrong.)