This is the way Termux works so far, External storage: Storage on external SD cards. Each app has a private folder on the external SD card, and interchange between them needs to use a special API not yet available in Termux.
I don't see anything in that link that says that sdcard (outside private folder) is read-only.
The way I read "Each app has a private folder on the external SD card, and interchange between them needs to use a special API not yet available in Termux." is that I can't read another applications private folder, but I'm not trying to - I just want (write+delete) access to the publicly accessible parts of the sdcard (lots of other applications do this, and termux did in Android 7).