The link you gave is not a sample problem guide. It is a unit test file. The current sample problems can be accessed from the PG problem editor in webwork2. There is a button above the code editor window for that.
I just did a lot of work on the sample problems in a pending pull request (see https://github.com/openwebwork/pg/pull/1152). There are several problems that demonstrate the new PGML niceTables syntax.