From the O365 Portal you get what is called "click to run" - which installs via a streaming service. It basically streams the application down to you - and you can actually start using it before it's installed completely.
As you uncovered another versions is what is called MSI based - this is the traditional install method for windows applications. Here - you have to download the entire package and then install it before you can use office.
Technically - there are ways to get these both on your system, but it's not a good idea which is why Microsoft blocks you. When you have both on your system some things stop working - particularly with the integration with online products or web based products. For example - trying to open or save directly to O365 from an application wouldn't work and you would see error messages.
For anyone still searching for an answer on this, I found Microsoft have provided installers that worked for me in this situation (Windows 10, O365 version of Office 2016, click to run install error):