It isn’t a Pre compare script

When you use SSDT to deploy your dacpac, a pre deployment script lets you run some T-SQL scripts before the script which brings your database up to date with your dacpac.

The important thing to note is that the process is:

sqlpackage compares the dacpac to the database sqlpackage generates a list of changes to make sqlpackage generates the T-SQL statements to make those changes sql package either creates a script for later execution or deploys the changes The script that is created *or* is executed contains, 0 or 1 pre deployment scripts, the statements to generate the required changes and then 0 or 1 post deployment scripts.