C# and C++ are also industry-standard languages. The point also applies to them, in the sense that those are also better languages than tool-specific languages - which it seems you agree with. It's talking about the disadvantages of tool-specific programming languages like GDScript and GML where that programming language is only used by one piece of software.
For example, if you want to write some server-side code, you'll have a much easier time with an industry-standard language - C# and JavaScript in particular are great for that (ASP.NET, node.js etc). Suppose you want to branch out in to some other part of the tech world - those skills are far more useful and have far more jobs available than any tool-specific language.
Education is also an important part of the business for us, and so we're also posing the question to teachers: do you want your students to learn an industry-standard language they can directly get a job working with? Or will they be learning some tool-specific language that won't provide as easily transferable skills?
Perhaps you think C++ and C# are better languages than JavaScript for game development, but that's not the point we're making: I'd also say C++ and C# are much better options than tool-specific languages, for the same reasons.
For me "don't invent your own programming language" is the hill I will die on - I think it's a big strategic mistake, even though it seems lots of people disagree. We're making the point that for developers writing code, there are a lot of benefits to using an industry-standard language, be it C++, C#, JavaScript, or any other major existing language.