Yes, it is true that many jobs are mainly for Wordpress.
I don't know if Drupal is nearly as popular outside the United States as in the country. It seems that the US government gave it a boost in relevance, but other than that it seems it is not as popular as Wordpress.
Other systems have some popularity. There is demand for PHP developers that is not specifically for those PHP products, but if you have the experience to work with them, it is probably better than having a certification in the language. Some frameworks have also some demand.
Take a look at the skills required in the jobs posted in the PHP Classes site:
phpclasses.org/jobs/