Explain the problem.
In the documentation, the variable beamerarticle is under the beamer section of variables. However, in the templates, it is only loaded if the output type is latex (article), not beamer. So really this variable should be under the latex variables. If the option beamerarticle is given when the output format is given as beamer, it does nothing. If the variable is given with the output format as latex, then you get what you want.

Pandoc version?
Pandoc 2.11.12 on ubuntu 18.04 LTS