The article had me confused until I realized this was 2013. Even then it is out of date.
Muli-Page applications have had business logic on the client-side for years as a technique to reduce the feeling of latency since at least 2005.
If by "Traditional" web apps you mean web apps from the mid to late 90's, sure it was done entirely with POST and no business logic at all client side.
At the very least data validation has been being done client side for s long as Javascript has existed. (and also server side as a second level of checks if you care about data integrity and security)
SPAs did not appear overnight. They are a logic evolution of what has been happening for years.
> sure it was done entirely with POST and no business logic at all client side
Even if you don't have business logic on the client side, you still have a state: tab position, state of controls, etc. These are not part of the data you really care to store on the server, and yet they have to be sent there at each and every page reload, as they might be needed in order to build the response.
Muli-Page applications have had business logic on the client-side for years as a technique to reduce the feeling of latency since at least 2005.
If by "Traditional" web apps you mean web apps from the mid to late 90's, sure it was done entirely with POST and no business logic at all client side.
At the very least data validation has been being done client side for s long as Javascript has existed. (and also server side as a second level of checks if you care about data integrity and security)
SPAs did not appear overnight. They are a logic evolution of what has been happening for years.