It is specific to postgresql in the sense that it supports everything that posgresql does : sql extensions ("create table like..."), its procedural language pl/pgsql, foreign data wrappers, table partitioning, among many many others ; and these are usually totally incompatible with other RDBMS.
But the main developper is studying closely an integration with pgloader, which would make such a compatibility a thing in a near future.
If you stick to the pure design features of pgmodeler, "keep it classic" and never go for an implementation, then pgmodeler is somehow universal.
Edit : To answer more precisely, the model is "universal", the code produced when you export the model to a database is specific to postgresql (sql data types, extensions...).