It’s debatable. Everything is layered, and one layer’s application/product is the higher layer’s tool/platform.
In the context of understanding where and why OSS is dominant, I think the tool/app distinction is whether the thing solves a software problem or whether it solves a business problem.
Through that lens, Photoshop is an application, while VSCode is a tool.
So if we turn the clock back ten years, Visual Studio (the behemoth sold by MS to C# and C++ developers) would be an application, wouldn't it? It solves a clear business need, otherwise why would businesses pay so much money for it (more than for Photoshop). MSSQL and Oracle DBMS clearly solve business problems, they have slick UIs and sale people with powerpoint decks. Perforce P4 is an application that solves business needs for creatives (and also comes with P4V for a slick UI and costs photoshop-level money)
But at the same time Visual Studio is in the same category of software as VS Code, Oracle DBMS in the same category as postgresql, Perforce P4 in the same category as git. Surely that can't be it?
I'd agree in a heartbeat that developers are better at solving problems for developers. The less disconnect there is between developer and customer the better development goes, and the disconnect doesn't get lower than building developer tooling. But those things aren't any less apps or more tools than the things artists or technical writers or accountants use
Why is not Photoshop considered a tool? You use it to [...], as a tool. But then I do not see the difference between application, like an application can be a tool, can it not? It makes my head spin and English is not my native language.
Even the qualifier "end user" application doesn't seem obvious to me. Is Photoshop an end user application or a tool for artists?