The documentation includes the following in the Remarks section:

Non-zero exit codes are set only when you run Interactive SQL in batch mode (with a command line that contains a SQL statement or the name of a script file).

This also might be applicable:

For Windows, there are two executables:

1. Batch scripts should call dbisql or dbisql.com, not dbisql.exe. The dbisql.com executable is linked as a console application.

2. The dbisql.exe executable is linked as a windowed application and does not block the command shell from which it was started. If dbisql.exe is run from a batch file, you won't see any output sent to the standard output or standard error files.

In addition to Graham's suggestions, you might also use the DBBACKUP utility directly if your script does not contain more than the BACKUP statement.

Like all other database utilities, DBBACKUP returns exit codes that you can check for:

Software component exit codes