Well with the MacOS 10.14.6 beta that was released yesterday, this changes everything. They always push a MacOS update around the same time iOS/WatchOS/TvOS ship. Since they seeded another beta for MacOS, obviously they need at least another week of testing MacOS. I would think MacOS 10.14.6 is required for Apple Card because you are asked to setup Apple Pay on Touch-ID Macs during the setup process. Some client-side software is required.
I say Apple Card launches next week with 12.4. iOS 13 beta 4 today, and us beta testers on iOS 13 will be able to start to use Apple Card starting with beta 5.
This makes sense to me. I also found it odd that they released the MacOS update on its own.... Not even tvOS or WatchOS. I think your reasoning gives a plausible explanation for it. Although I am little bit skeptical about beta 13 testers getting access to the card first to be honest, but you never know.