Well Cav is known to favour a long stem, so this will compensate a little for the frame being theoretically too small for him (and may add enough weight to make the stem heavier than other options). My guess is that he likes the low front end but, as the article says, he may change back in a couple of weeks.
Just been on the Zipp website - claimed weight for the Zipp SL Sprint stem is 165g for a 100mm stem.
Granted this is fairly light - but 100-110 are 'probably' considered average/standard stem lengths as supplied on new bikes to the public & so if Cav 'rides a long stem' (insert smutty humour) it will weigh fractionally more.
Not a problem as the UCI has a total weight limit of 6.7 kg (could be 6.9) on complete bike weight anyway.
The point I am making is that the SL moniker for a Carbon/Titanium built stem that is not SuperLight is a bit of a joke (all though it does look strong for sprint purposes).
Again my aluminium/titanium 3T stem is approx 130-135g.
Add length to anything though and surely you have more flex (not good for sprinting).
Still can't get my head round that frame size though.
The 49'' top tube must be a lot smaller than a 52" & so you would need a very long stem to compensate.
The geometry in relation to his body size must be way out.
No expert as I say - just seems like Madness.