That's true, the problem with the original statement is that it is too broadly scoped by "knowledge", implying that it is any and all knowledge. If I obtain the knowledge to write a singleton in object oriented programming while at work - even if the concept is applied to a work project, and later use the programming concept of a singleton in my own software, then they can't do shit.
A simpler example that shows that it's too broadly scoped is that if I get trained and certified to use a forklift for a job, and later start my own company and have to use a forklift, there is no precedent for my original employer to come after me for using a forklift in my business operation just because I learned how to use a forklift while I worked for them.
If the knowledge is proprietary or copyrighted or a trade secret and what I do uses any of that, or what I produce is a 1 to 1 product of that, then they can come after me.