Ah, sorry about that, this was a bug in the recent nightly release! I'm just building a new version.
In the meantime, the actual fix needed is to find the following line in recipes/train.py and change the "t" to "s" (the shortcut used for this particular argument, a copy-paste mistake caused a conflict here):
"senter": ("Comma-separated sentence datasets", "option", "s", split_string),
Edit: Fixed in v1.11.0a7!