Alternately it is all maintenance.

I think that it is important to keep in the mind the context of the software being developed. For example, developing software for Cochlear hearing implants is considerably different to developing a web application.

Sometimes some words just don't apply to what you, yourself, are doing. And we should resist the urge to redefine words to match what we are doing. To paraphrase Brie Larson, "that word is not meant for you".

I would say that "Software Maintenance" doesn't relate to, say, web development as much as it does to other types of software because often -- especially when developing bespoke systems for clients -- there are additions based on changing business requirements.

But that said, there are activities that are best classed as "Software Maintenance". These are often changes required because of changes to software dependencies or 3rd party services. I agree with the article -- I think -- in that additional features shouldn't really be classed as maintenance.

That all said, I think the main point of Software Development vs Software Maintenance -- especially when discussing the Waterfall model of development -- is that there should be an expectation of ongoing cost after the software is initially considered finished.