Your addon looks truly awesome. I definitely want to add it.
Their addon may be different because the webservice allows them to access a constantly updated centralized database and, possibly, a machine learning algorithm based on that database. It looks like it might be more similar to Akismet.
For example, I found out about cleantalk by googling the ISP of a spammer that hit me this morning. I found the following awesome page from Cleantalk, the addon we are discussing: https://cleantalk.org/blacklists/AS55286
Apparently, those IP numbers have been used heavily by spammers recently. Because the info came from Cleantalk, their engineers can use the info to update their algorithms.
With that said, your addon rocks. I like that I could use it to customize my defenses for the specific spammers that are attacking my forum (i.e. their ISPs, GeoIP info, terms, etc.)
Personally, I'll probably go for Akismet because it's in the core and yours because it's configurable, but I was surprised that no one
You can score by AS number in my addon as well, the defaults cover most spammers, and manually adding new ones is easy, if you look at registrations that made it through (spam trigger log). I add like one AS once a month in our 3M posts forum. If you have entries to contribute, send me a PM anytime.
Akismet is for new posts (checks content), my addon works at registration time, so there is no content it could check. So you definitely want to use both.
They appear to be a Russian firm, so a bit of skepticism is natural. On the other hand, I'm pretty happy with my Kaspersky Internet Security (though I wouldn't use it if I worked in the state department )
I'd like to answer my own question, having come up with the answer while writing an unrelated post.
If a addon isn't listed in the Resource Manager, it means that it may have been developed in an unlicensed copy of XenForo. If a company can't be bothered to pay licensing fees like the rest of us, then how can we trust them with their data? Now I understand why having an addon not listed in Resources would be a bit of a red flag, so thank you for calling that to my attention.
I wish I had put two and two together earlier. Perhaps they are "fly-by-night" after all.
After making my last post, I went to the cleantalk.org forums and made a post in which sums up how I feel: "Takeup of your product among XenForo admins might be higher if you reached out to them by adding your product to their Resource Manager. You can only do this if you have a licensed copy of the software, but I certainly hope that your development copy is licensed. I suspect I speak for other XenForo admins when I say that if we can't trust you to pay the very inexpensive and reasonable licensing fees that the rest of us pay, then we will never trust you with our data. There are too many other great products out there."
I'm very glad to see their product listed here now. All's well that ends well, right?