Except that isn't supported by the (right now, admittedly anecdotal, as I don't have any logs from 1½ years ago when we did it) evidence. At level 80, you could still hit TBC bosses in TBC gear at the expected amounts for the boss being lvl 73. The same was true for defense - you didn't need more defense than you did at 70 (before defense was removed at 4.0.1). It also would be a very weird mechanic to single out in regards to hit/miss and no other level influenced statistics. Each boss (group, type, individual) had all those stats coded to be level 73 - as supported by the evidence gathered throughout the years, which we have based all our theorycrafting numbers on.
This is getting a bit off topic, I think, so I'll just round it off by saying, skull mobs are considered the content level +3 as of now (and arguably always have been) - target dummies are considered for the content they have most recently been released in - there are now "Northrenders training dummies" clearly indicating that the dummies have been upgraded to cataclysm.