Yes it is due to var limitations.
My guess is the functions already existed in WDL/C-Script, where there were only var and string.

I fixed this in a dirty way for end of file detection and otherwise avoided usage of file_seek and file_length as much as possible.

#define feof(f) (file_seek(f,1,4) == file_length(f))

if both functions deliver the identical (wrong) value, we've most likely reached end of file