As I understand, Cheerp is a tool in a similar way it is GCC, licensed with GPLv2. The license applies to the work reached by the copy rights, but not for use restrictions.
So, as long as one does not distribute it (as code or object) with the programs created, the resulting program is not restricted.
My confusion arieses with the text in the page:
As parts of Cheerp are licensed under GPLv2, a copyleft license, if you distribute your application you must provide your users access to its sources under the same license.
That is true if the application is a derivative work of a GPLv2 work, Cheerps in this case. But it is not the case.
So, the following addition looks like a violation of GPLv2, because adds up use restrictions to the license:
If you are using Cheerp for developing closed-source, commercial applications you must purchase a commercial license.
And if you don't distribute a GPLv2 program with the same license (pure GPLv2, without additional restrictions), you are violating the license.
TL;DR: I think that you can't add a use restriction to a GPLv2 licensed program.