the mechanics have not changed. ditto transforms and takes any stat boosts this is true. however choice scarf, band and specs are not considered stat boosts as you never see the text say "so-and-so's attack raised.". just like the double speed from Unburden cannot be baton passed as it isnt actually raising your speed by boosts, it is just doubling your speed overall.Apparantly, Ditto actually copies the stat boosts of items in gen 4.(as seen in the bold)
If it did completely copy stats, then Ditto would outspeed the Hihidaruma, but It didn't meaning the transformation mechanics were tweaked.
So lets say I have Hihidaruma had 200 speed + Scarf (300 speed in total). Ditto would copy the 200 speed stat, but NOT the scarf boost. It needs to use it's OWN scarf to tie with Hihidaruma, and that's why I think Hihidaruma outsped him.
so here is what i figure. boosts such as Choice Scarf, Choice Band, Choice Specs, DeepSeaTooth, DeepSeaScale, Light Ball, Unburden boost, Thick Club, and any type of boost that doesn't specifically increase a stat by +1, +2, etc. is not copied by Transform and will only receive those boosts if that pokemon meets those qualifications as well. for example: if your ditto has a Thick Club equipped and transforms into an enemy Marowak or if Ditto has a Choic Scarf and transforms into Hihidurama.
Transform does exactly what the description says. it transforms into the pokemon with all of its Stat Boosts intact, and items like choice scarf and abilities like unburden do not count as boosts so they are not factored in (unless of course ditto uses his item after he transforms into an unburden pokemon). i hope that makes sense?