Marketing: noun, the action or business of promoting and selling products or services, including market research and advertising.
The keywords there are "action", and "promoting". The first 4 items you list are not marketing. They are preparing for the market. The only thing there that is actually marketing is "They increased awareness and trial of their product via word-of-mouth" and that only barely counts because most of it is OTHER people marketing it for you, without your consent, control, or even input. So, it's not Duolingo marketing themselves. It's Duolingo's users marketing it for them. That's usually referred to as Evangelism and Guy Kawasaki has made his career out of explaining and promoting that idea http://guykawasaki.com/
You - and the article - are conflating advertising with marketing. From the man who literally wrote the book on marketing management[0]:
"Marketing is the science and art of exploring, creating, and delivering value to satisfy the needs of a target market at a profit. Marketing identifies unfulfilled needs and desires. It defines, measures and quantifies the size of the identified market and the profit potential. It pinpoints which segments the company is capable of serving best and it designs and promotes the appropriate products and services."
I.e. advertising and promotions are just one part of marketing.
The keywords there are "action", and "promoting". The first 4 items you list are not marketing. They are preparing for the market. The only thing there that is actually marketing is "They increased awareness and trial of their product via word-of-mouth" and that only barely counts because most of it is OTHER people marketing it for you, without your consent, control, or even input. So, it's not Duolingo marketing themselves. It's Duolingo's users marketing it for them. That's usually referred to as Evangelism and Guy Kawasaki has made his career out of explaining and promoting that idea http://guykawasaki.com/