Just my two cents with the caveat that I am a lawyer, recently retired, but I did previous work in tort areas so I understand the law of defamation (slander, libel, slander per se and libel per se). The important things to know about defamation are that (1) if it involves a public figure, it is significantly harder to prove, (2) defamation only involves claims of provable or disprovable facts -- opinions no matter how thinly stated are not defamation as long as they are not based upon provable or disprovable facts (for example saying you hate something is not defamation unless you say you hate it because of a specific provable or disprovable fact), and (3) truth is always an absolute defense.
I'm glad that @james633 posted the follow up video and I'm late to this party but I watched most of that video and think this is a lot to do about almost nothing.
No litigation has been initiated as far as I know and Erin was having a cow due to the threats because he is a normal human being and didn't expect that kind of reaction to a generally positive review. Erik overreacted and acted like a bully/jerk for no real reason that I can see. He also impliedly blamed Erin for loss of sales probably without any kind of proof but just using it as a threat.
Certainly Erik could have reached out civilly to Erin and discussed any suggestions or concerns he had and it appears Erin would have taken such communication seriously. Instead, Erik has created an uproar and cast himself in a negative light.
There is no real defamation case that I can see (and even cases with good proof of defamation are usually not good cases unless they involve outrageous conduct -- see recent news on those types of cases). Even if Erik could show different measurement results, Erin's results, if not faked, are provable facts and therefore truth -- absolute defense. Any such case filed against Erin would be dismissed long before any trial. If filed in an anti-SLAPP state, attorney's fees would be awarded but hopefully Erik was just making an empty threat.
I hope this all just simmers down and the YouTubers see there was really no real threat to their livelihood.