Yeah, they have a newsletter swap feature that I don't use either and I agree it's confusing.
I think their newsletter magnets promos are better than the retailer promos, though, I'd say almost as effective as the ones on BookFunne. If you're not there you should try because there are quite a few fantasy and YA promos.
I'm trying this KU for the first time but I'm not sure if it's effective. It's at least extra exposure, I guess.
Also, I must say that I don't love their creation tools. Now that you're saying that I considered removing the question about mailing list, but there's no place to edit it. There's also no way to edit the start and end date. So BookFunnel is still better.
That said, it's good to use different services, because then you cross promote with a greater variety of authors.
The point about interacting with a different group of authors is a good one, as I imagine everyone isn't on BookFunnel.
There's nothing wrong with asking about mailing list size and when the promo will be shared. BookFunnel now asks for the latter information, and giveaways there sometimes require a specific mailing list size, so it's really not a problem. Story Origin just needs to set that up as a separate question rather than having it branch off the newsletter swap one. It took me a little while to realize I wasn't inadvertently creating a newsletter swap.

Story Origin is a new service (and free at the moment), so it's not surprising there are some confusing things. I was looking around for a mailing list builder, and apparently there is no separate label for that. I guess the giveaways are for that, but I checked a couple of the applications, and it isn't clear. There's no question about whether I want to collect email addresses or not, though one of the authors did say that was an option in the directions.
If the mailing list builders are almost as good as BookFunnel's, people should definitely try them. In January, after I'd nuked my mailing list over GDPR, it was at 15 subscribers. Now I'm at 1,603, 46% of which have come from BookFunnel. My open and click rates are good. I suspect Story Origin may have at least some readers who aren't on BookFunnel, so it's definitely worth trying it out.
I previewed your giveaway page, and it looks good, maybe even a little better than BookFunnel's.