I also have to warn readers not to trust this review. This is poorly researched and written at best, and amateur journalism at worst. To echo what the above posters wrote, you must always test software on comparable devices and invest in equally proprietary services for each to remain objective and be taken seriously.
I'm going to give you the benefit of the doubt and assume this was quick and dirty test without the benefit of proper peer review. Here's some advice for next time (and other reviews in general):
1) You cannot call yourself a serious tech reviewer without subscribing to all the brand services and testing integration with compatible products. That means you MUST subscribe to APM, GPM, and Apple Music to be objective, and perhaps the most popular service in this category BY FAR, Spotify. Also, test Google and Alexa by syncing them with android devices, not an iPhone. Actually that doesn't even matter if you do your research, since both the Google Home and Alexda apps can be used to setup their assistant with whatever music app you want.
2) You cannot cannot cannot compare apples (home devives) to oranges (iPhones) and come up with anything but useless, biased results. Of course Siri is going to work better for making calls and texts on a phone compared to anything on a smart speaker. Why not use Google and Alexa on a phone for this test instead? They are both available on any android phone with Marshmallow or higher. Even better, dock Siri serious points for not being able to do anything without a phone while the others can do all of that on a phone, plus some of it on home devices. This will of course change when HomePod arrives.
Also, Google Home can absolutely send driving directions to any device using Google Maps with a Google account.
3) The top 3 assistants are not Siri, Alexa, and Google, at least not definitively. Cortana?