"If you were a completely unknown couple to the judges your chances of making it to the next round would be slim."
Not true at all.
Quality dancing captures the eye and draws attention to itself. That 7.5 seconds figure is not relevant, because judges do not give each couple a fixed amount of attention when making recalls under ordinary circumstances. Instead, they look for the requisite number of promising couples who stand out from the others.
This isn't about detailed evaluation, it's about holistic perception: that couple looks strong, vs that one looks weak. All of the mentioned factors feed into this perception, but not specifically - there's no ok, feet okay, topline okay, timing okay, checklist, it's just a comprehensive "promising" or not.
(There is an exception: extremely experienced judges in huge comps can have a sound enough idea of the standard for entry to the next round that they can basically accept or reject each couple as they pass by, without having to compare them to others)