Use Robocopy to copy only changed files?

To answer all your questions:

Can I use ROBOCOPY for this?

Yes, RC should fit your requirements (simplicity, only copy what needed)

What exactly does it mean to exclude?

It will exclude copying – RC calls it skipping

Would the /XO option copy only newer files, not files of the same age?

Yes, RC will only copy newer files. Files of the same age will be skipped.

(the correct command would be robocopy C:\SourceFolder D:\DestinationFolder ABC.dll /XO)

Maybe in your case using the /MIR option could be useful. In general RC is rather targeted at directories and directory trees than single files.