Binary Ninja is a fine piece of software, but it is more ethical to advertise this article as "nice reversing tutorial included with said software", because not-so-hidden shameless advertisement for it is worse.
With that said there is nothing stopping people from starting with radare2 if they wish to, there is a lot of great tutorials for it available online. But in this case the recommendation of using Binary Ninja was one which the author (Nitrax) made because "due to its low cost .. compared to the functionalities provided", the author even further added "A demo version is available for free and should be enough for beginners."
I can understand the insistence on free and open-source software, but a lot of OSS tools in many fields have simply not caught up to their paid counterparts in all aspects. And to me, it feels much like calling someone a shill for saying "Make sure you use a good drill for this" instead of supplying references to a free & open 3D printable drill schematic.