My feeling is that you are getting overly specific about requiring every detail to be an exact match for a similarity to hold. I don't think that is reasonable. Both the criminal subsets of BLM and the brownshirts created an atmosphere of fear. In Seattle, people are afraid to voice their political opinions freely, whether online or in social circles or at work or in political action or protests, because of the threat of these groups and their aggressive tactics. That is what makes this comparison valid, ultimately, since these actions constitute terrorism.
Some other responses to specific things you mentioned:
> loosely organised group
BLM is a nationwide (and global) organization with chapters in numerous cities across America. Protesters in different cities share tactics, chants, demands, etc with each other. Many cities even share the same exact participants. For example, some people from Seattle were arrested in Kenosha recently, as they were using their food truck "Riot Kitchen" as a cover for planned criminal activity (https://thepostmillennial.com/portlands-riot-kitchen-arreste...). Just because these groups have some degree of decentralization doesn't mean they are "local" or just "loosely organized", and it does not reduce the chilling effect they are having on political discourse.
> somewhat indiscriminate rioters
The various incidents I quoted in my earlier comment were highly-targeted, which is the opposite of indiscriminate.
> para-military organisation that targets one specific ethnicity and went on to commit mass murder of said ethnicity on an unprecedented, industrial scale
There is no mass murder from BLM. However, this was not the primary purpose of the brownshirts either. The regime that the brownshirts enabled was responsible for significant mass murder, but the brownshirts themselves are described in Wikipedia (https://en.wikipedia.org/wiki/Sturmabteilung) as follows:
"Its primary purposes were providing protection for Nazi rallies and assemblies; disrupting the meetings of opposing parties; fighting against the paramilitary units of the opposing parties [...]; and intimidating Romani, trade unionists, and especially Jews such as during the 1933 Nazi boycott of Jewish businesses"
The parallels I am noting are things like providing protection for riots/criminal activity, disrupting opposing parties, and intimidation (of politicians, of private citizens, of businesses). My evidence is the long list of incidents I mentioned in my earlier comment. One of those incidents was even explicitly targeting an ethnicity. Another incident specifically carried misogynist tones. All of these incidents targeted people based on political views and created a sense of fear, so that those who harbor a different opinion are afraid to speak out. That is happening on an unprecedented, industrial scale, across virtually every city in the country.
> let me assure you that comparing riots among BLM protesters to the Kristallnacht is utterly unappropriate
Let's take a look at https://en.wikipedia.org/wiki/Kristallnacht. It characterizes the event as involving "Pogrom, looting, arson, mass arrests". The definition of pogrom is noted in its own Wikipedia article as "A pogrom is a violent riot aimed at the massacre or expulsion of an ethnic or religious group, particularly one aimed at Jews". I mentioned there have been incidents of mobs showing up at the homes of white citizens and demanding they leave - so that checks the "expulsion of an ethnic group" part. We obviously have also had violent riots, looting, and arson as well - there are tons of videos showing that. To my knowledge, city leadership did nothing to stop many of these incidents, by the way.
Seattle did not have the scale of Kristallnacht, nor the mass arrests/incarceration, nor the murder. I don't think Cliff Mass was claiming that, and I don't think readers interpreted his comparison that way either. But since Seattle has had many of the other elements, I felt the comparison was valid-enough to draw.