Yes, you are correct, I should have answered your question sooner.
To clarify this (someone else may find this useful) the premium services provide by Sucuri (the ones that require a subscription) are not related with this plugin (with the exception of CloudProxy, will explain below). The plugin is developed, maintained and released for free and we in essence use it to spread the word through the ads shown in the right side of the plugin interface (which by the way can be hidden using an option available in the settings page).
I am not a business guy so my skills to sale a service/product are very raw, if anyone wants to have more information about these premium services I suggest you (all) to use the chat available in the bottom-right corner of the Sucuri’s main website. Read the customer reviews [1] and the features with prices [2].
The relation between CloudProxy and the plugin is based in the technical side of the firewall, some 3rd-party programs require to have the real IP address of the visitors, since all the connections sent to the website are passing through the firewall first (to filter malicious data) the IP address is moved to a different variable which is fixed by the plugin. When people purchases CloudProxy we generally recommend to activate the “Firewall WAF” page with the API key available in the Sucuri dashboard to fix this thing with the IP addresses, which is something common in other firewalls offered by other companies.
That is basically the only thing strictly tied to a Sucuri subscription, in theory you could even write your own API service and point the plugin to talk to that instead of Sucuri’s, which would make the plugin a completely independent piece of software. I actually wonder why no one have done this before, anyway if you need more information let me know.
[1] https://sucuri.net/customers/
[2] https://sucuri.net/website-antivirus/signup