AFAICT, you need to first click on the link to install the app, taking the first step in that direction. But then close whatever app install window that opens, return to the invitation page, and the Join From Browser link should have appeared.

Basically, the option to join from a browser is only shown once you first signal intent to install the app. This procedure is described in the Zoom docs:

https://support.zoom.us/hc/en-us/articles/214629443-Zoom-web...

The meeting host can also change a setting that allows the Join From Browser link to be displayed without having to go through the procedure above:

https://support.zoom.us/hc/en-us/articles/115005666383-Show-...