SWAPFILESIZE value is a number. If you put "other", you would just make swap creation command to error out.
If you need more memory on your box, get more memory. Don't treat swap as "temporally" memory that you will be using permanently by ignoring performance hit that you get there.
Instructions that you linked tell you where that swap file is. Technically they are probably for older LibreElec as current swapfile location is configurable. Remember that LibreElec is not regular Linux, you can't use Ubuntu instructions on LibreElec without understanding what they do and you can't put random stuff in its root file system.
If 256MB swap is improvement over 128MB, remember how much memory your cell phone has and find some dimes in your couch for real memory.