I don't agree with that. Once you understand the basics of building performant databases/queries, that knowledge will persist across databases. There are small differences in syntax and features, but any remotely competent person should be able to pick those up while they go, provided they understand what happens under the hood when they execute SQL.
In what way are they wildly different? I spent 15 years working in Oracle and the last 2 years working in Postgres. They are slightly different (Postgres adheres to the standard much more closely than Oracle), but 95% of my knowledge of SQL transferred from one to the other.
A simple example would be "how to select groupwise maximum" where Postgres would respond well to a correlated subquery and Oracle would respond well to a window function and derived table. The engines and optimizations are different.