Whilst that observation is correct, even if a file is not marked Read only, it can still be "in use by another person or program". Read only status will not stop you manually deleting a file, it will simply query whether you really want to do it.
Manual delete in "Safe mode" should work with most "in use" files (except perhaps some system files) because the parent program that would normally be using them would not generally be running in safe mode. particularly if it is a games program. I did sugest you try manual deletion in post 10 in this thread, but I can't find any subsequent post indicating you actually tried it.
The question may be whether these files are needed for the owner program to work properly, even if they have been deposited in an area intended only for transient files, so be warned, deleting them may have a negative effect on their parent program.