Couple thoughts:
1) How many couples do you know of that claim that practice schedule? When American Dancer interviewed some US finalists / champions leading up to nationals, I can't recall any claiming more than ~3 hours of practice a day, maybe more on weekends.
2) These are almost certainly self-reported numbers. People are going to fall into a social desirability trap when they self-report, meaning they're almost always overestimating their typical practice amount. This, of course, assumes that in their social circle (elite dancers) more practice time is socially desirable.
3) 5-6 hours of practice is not totally impossible on an 8-5 schedule, especially if you assume only 6-7 hours are needed for sleep and you have reasonably short travel times and allow for meals, etc. Also, a lot of folks don't work an 8-5 schedule, and some actually work less.
4) Some younger amateur couples might be doing the absolute bare minimum amount of teaching to afford everything (possibly with aggressively reduced living expenses) or may be benefiting from familial support (when you consider how much 4 years at a university can cost, putting the comparable amount towards a child's dance education post-high school seems a lot less crazy).
ETA: Also, you should consider if that amount of practice time is truly helpful or needed. My coach and his partner (top 96 Open Latin at Blackpool last year, so probably a really good couple) won't practice more than 3 hours at a crack before taking a substantial break (on a weekend of training, they frequently do more than one practice in a day). Effective practice is both mentally and physically exhausting, so sometimes less is more.