I found that if you have
args file
args.txt
video.mp4
-Subject=MySubject
-api LargeFileSupport=1
-overwrite_original_in_place
-m
-execute
$ exiftool -@ args.txt
Warning: Tag 'api' is not defined
Error: End of processing at large atom (LargeFileSupport not enabled) - video.mp4
0 image files updated
1 files weren't updated due to errors
But documentation says
QuoteArguments in ARGFILE behave exactly the same as if they were entered at the location of the -@ option on the command line, with the exception that the -config and -common_args options may not be used in an ARGFILE.
It doesn't mention
-api settings
From the docs on the -@ (Argfile) option (https://exiftool.org/exiftool_pod.html#ARGFILE) (emphasis mine)
The file contains one argument per line (NOT one option per line -- some options require additional arguments, and all arguments must be placed on separate lines).
Change it to
-api
LargeFileSupport=1
@StarGeek, thanks it worked!