From what I can tell, the article you linked talked mostly about the illegality of concealed carrying a fixed blade knife. From what I can tell, it's perfectly legal to carry a folder as long as it doesn't possess any "bad" features[2] and is within the length restriction.
The law in Seattle is pretty clear.
> "Dangerous knife" means any fixed-blade knife and any other knife having a blade more than 3 1/2 inches in length[1].
___
1. https://library.municode.com/wa/seattle/codes/municipal_code...
2. My understanding is that a lot of otherwise normal "folding knives" fall under the "gravity knife" definition because the detent that holds the blade closes is often pretty weak (on purpose - it makes opening the knives one handed easier).
In Colorado I believe the maximum length allowed is 3.5 inches in blade length. This sounds simple but can even be a grey area because some knives like Spyderco have a part of the blade thats not sharpened and technically part of the handle.
NYC used to be very knife unfriendly and you could get charged with carrying a gravity knife if the knife could be flicked open in any way. Good news though as it sounds like that changed https://reason.com/2019/03/28/federal-judge-rules-new-yorks-... There was still a case in NYC of a chef who was charged with something for carrying a very small pocket knife on his way home from work. He was charged with something for the pocket knife but from what I remember it was totally okay that he was carrying a bunch of large chefs knives in his backpack.
Its important to read and understand the law as well as how its actually enforced.
I went down a rabbit hole about EDC in WA state. There is a grey area that carrying a pocket knife requires a concealed weapons permit.
Most folding knives have a pocket clip, but as far as I have found, it's unclear if that in an of itself would qualify as not concealing the weapon.
https://www.akti.org/state-knife-laws/washington/