Yes, the ecosystem is constantly changing - but at any given moment in time, you have a specific product targeting a specific market. PMF is a framework to see if you're delivering on your promise to that market in that specific point in time. The corollary to PMF is that the market you've picked is big enough and/or growing.
PMF is not going to save you from external forces (the arrival of the internet in yesteryears or in more contemporary times, the SaaS-ification of everything) happening and neither does it claim to be. That would be an entire different topic ("Innovators Dilemma" - but even then you need to have PMF in the first place to even have that dilemma ;)
A journey of a startup to scale is better categorized as a continuous series of PMF points where the product evolves to target a continuously changing perception of the target market in the eyes of the founders.
Obsessing over PMF as a fundamental metric will limit founders from exploring the true complexity of the ecosystem in which a startup operates in and can often be a stumbling block in optimising for their subjective outcomes.
But rather than ruminating at length on "PMF", I just prefer saying "if people ain't buying, change what you're selling".
On the other hand, I can see how it's conceptually more relevant to companies taking the route of "raise capital first, get exposure second and figure out profitability later".
Startups exist in a complex ecosystem of competing forces. This ecosystem changes all the time. Your product and the market are only two of the multiple factors that could influence your startup. Even then, what constitutes your product and how you define your market will also change throughout the course of your startup.
The ideas behind 'product-market' reduce this complexity into an intellectual exercise whose premise that product should fit market for success to happen is too simplistic and inaccurate. Assuming the product-market fit theory and extrapolating to corollary's and model's the kind this article portrays is great for an MBA classroom workshop but little else outside.