According to the manpage:
An optional list of archive members to be excluded from process‐
ing. Since wildcard characters normally match (`/') directory
separators (for exceptions see the option -W), this option may be
used to exclude any files that are in subdirectories. For exam‐
ple, ``unzip foo *.[ch] -x */*'' would extract all C source files
in the main directory, but none in any subdirectories. Without
the -x option, all C source files in all directories within the
zipfile would be extracted.
So the answer is:
unzip -l <zipfile> -x '*/*'