I think line 3 of the help description addresses your question.
ExecuteSQL enables you to execute SQL SELECT statements containing dynamic parameters to safely query FileMaker Pro databases in order to avoid security vulnerabilities through injection attacks.
ExecuteSQL does not recognize relationships created in FileMaker Pro, which gives you flexibility to define relationships in SQL statements and retrieve data from any table, independent of the layout context.
ExecuteSQL cannot be used with SQL statements that modify data or the database schema (such as the Insert Into or Delete Table commands).
If an error occurs during query parsing or execution, FileMaker Pro returns ?.
Execute SQL script step is for querying external ODBC data source.
ExecuteSQL is function for querying Filemaker DB and does not support UPDATE, DELETE.
If you want to use UPDATE SQL command on Filemaker data, you can install SQL plugin: