Can Siri be instructed to scroll pages for me in Safari?
In my honest opinion, using Siri on the Mac is extremely frustrating and has huge limitations. Again, in my honest opinion, enabling Enhanced Dictation in System Preferences is the way to go. Not only do you have access to dozens of pre-existing Dictation Commands, you can also create your own custom Dictation Commands. 





Using Automator.app is a good place to start getting your feet wet with making new dictation commands.




So to answer your question, not only can you scroll using Dictation Commands, you can completely navigate through websites using Dictation Commands. 

For example, while using Safari if you were to speak the dictation command "Show Numbers", this is what you would see...

And if you wanted to click on the link "add a comment", under your original post, all you need to do is speak the actual number (69) which is in the little blue arrow pointing to that link