Hi I did some testing for you, here is my results.
First I just jumped in and switched off the modem and tried to load the game with my game icon on my desktop. This popped up.
https://i.imgur.com/nJlzDAg.png
So I switched it on and then looked for the part they are talking about above, I could not find it anywhere (I will look again) so I just saw the part where I could go offline and selected that instead.
https://i.imgur.com/N1vaqWF.png
After saying yes I am sure this page popped up
https://i.imgur.com/mimXa2F.png
I clicked on the little play button at sims 4 and got this
https://i.imgur.com/uQZymRj.png
Gosh well now I was confuzzled :/ but I switched on my internet again and started the game from that little play button, waited for it too completely load and then exited, thinking that it would now renew the license or do what is needed to make it work next time. Disconnected the internet again and got the exact same message. So now I put the internet on, clicked the play button and as the game loaded I quickly switched it off. The game kept loading but I got an error message in the game about the SDX not being able to download, I just exited out of the message and could load my game, I tried loading several different saves and they all worked as you can see below, I show you my internet is off.
https://i.imgur.com/gXxYWCJ.png
So all we need to figure out, is where to get that setting I got in the first error message, then it is supposed to work offline. I will see what I can find ;)