Silly me!
I searched my Mac and found all these epub files, without checking where exactly they are located, and wrongly assuming they are in a local folder. But they are somewhere on iCloud instead. That is why none of them worked directly, trying to open them in Kobo.
Unless I find out where I stored original epubs, before importing them to Books, they are lost for my new Kobo Clara, which is not good.
Is there any way to retrieve them from iCloud in a format that will be recognized by Kobo?
