A customer firing you might affect 2% of your income. A real boss firing you will cut 100% of your income.
Also, if you are the founder and CEO of a startup you have to answer to the board and investors, but there is much less power imbalance. Sometimes a CEO being fired is financially better than continuing to work. Think of the founder of WeWork.
“You always have a boss” is a bland phrase that doesn’t account for all the very important nuances of the matter.
Definitely not an “illusion”. There are very practical differences between the different types of bosses.
In an ideal SaaS scenario, your customer can cut off a tiny percentage of your revenue.
There are plenty of companies, SaaS or otherwise, in which a single "whale" customer makes up so much revenue that losing them would instantly make the company unprofitable. Similarly there are plenty of employees who can handle being fired perfectly well, financially speaking.