Those derivatives are, apart from not being really SQL (but a SQLish-like in the best case), only target an incredibly small subset of what is SQL.
When they say "we implement a subset of SQL" they really mean "we have a language similar to what used to be a small subset of SQL 92". And let me reinforce that 92, from the times of Windows 3.1. It cannot be compared to today's Modern SQL (http://use-the-index-luke.com/blog/2015-02/modern-sql) found on modern relational databases, like PostgreSQL.
When they say "we implement a subset of SQL" they really mean "we have a language similar to what used to be a small subset of SQL 92". And let me reinforce that 92, from the times of Windows 3.1. It cannot be compared to today's Modern SQL (http://use-the-index-luke.com/blog/2015-02/modern-sql) found on modern relational databases, like PostgreSQL.