I can recommend HeidiSQL, a useful GUI for MySQL, MariaDB, MSSQL and Postgres. https://en.wikipedia.org/wiki/HeidiSQL
They probably meant SQL Server Compact, which is a smaller package and comparable to SQLite. It was meant to power WinFS back in 2006. But it's closed source and now legacy.
There are many business applications that target SQL Server exclusively.
On the one hand, I find that highly annoying. On the other hand, even if you don't like Microsoft, SQL Server is fairly awesome, performance-wise and stability-wise.
But still, there are many applications where you cannot really choose what database engine you want to use.
I suppose PostgreSQL+FDW would be just as good as SQL Server Express+Linked Servers technically. You end up trading users' MSSQL experience for pgsql.
I wonder if it would be easy to support both backends and be able to switch between them depending on the query and which features and integrations you want.
Why choose a proprietary engine? Why not use PostgreSQL or even MySQL?