Hi @BambooCreative, welcome to the community!
Although I’m not familiar with the app you’re referring to, just note that many third party apps aren’t always listed in the Klaviyo app directory. Because Klaviyo lets anyone use their API to integrate with Klaviyo, many times the developers of apps just build and maintain their own integration with Klaviyo unilaterally. The apps in Klaviyo directory are usually vetted or built in collaboration with Klaviyo and pass the Klaviyo technical standards and requirements - sometimes even supported by Klaviyo directly.
This is not to say any apps not in the directory are not sound, it’s just they either chose not to be vetted by Klaviyo or have yet to do so. There are many fantastics apps that are not in the directory, and work fine with Klaviyo.
Hope that helps!