Maybe we're talking apples and oranges.
Your original note:
Pretty much all of the relay boards you find online have opto-isolators because they are all clones of the same design.
So we were talking generally until your latest post. What OP posted is an RC board, and I see no o.i. on it, though I didn't see its schematics. I'd recommend one in any case.
EDIT: re-edited whole post to reflect on what was being answered to me. Sorry for bringing this a little off-topic.