I think you are conflating marketing with advertising. Advertising is a subset of marketing, but wouldn't include things like:

- Writing clear and effective home pages (e.g. the Get Started button is the most prominent affordance on the https://reactjs.org/ homepage)

- The design of that homepage is marketing. Have you ever felt more comfortable using a product that has a nice website?

- Engineering blogs (e.g. the https://www.backblaze.com/blog/ seems to make it to the HN homepage every few months... they aren't just writing this stuff for fun)

- Making your product easy to get started with isn't strictly marketing, but letting people know that it's easy to get started with sure is (e.g. good documentation)

- Having a good pricing strategy is marketing. (e.g. make three tiers for different customers, but try to steer people to the middle one)

- Having a good pricing discount strategy is marketing. (e.g. AWS is basically free for students because they know who those students will want to use when they are making decisions in the work force in a few years)

A key thing is that not all marketing needs to be done by the marketing department. Marketing is about understanding the market and what will make them use your product. This is important even for software engineers.

Edit: fixed typo above and adding below. =========================================

Also, there are ways of getting the word out that aren't advertising but are done by the same sorts of people. For example, GoPro didn't buy online ads or TV commercials when they were a young company. They gave away GoPros to people doing crazy stuff and had them upload their footage to youtube.

Trader Joes doesn't buy advertising, but mixes up their inventory regularly in order to generate buzz. They'll even take popular items out of production, which I've always suspected is a marketing ploy meant to get people talking about them.

Lamborghini is another example of a company that doesn't make ads, but they fiercely guard their reputation. In fact, their marketing department is hiring right now for someone to manage their "car configurator" (https://configurator.lamborghini.com/), a product that they see as part of marketing.