having unexpectedly skidded on smooth concrete myself I think there are conditions under which the measured coefficients of friction do not apply. Clearly those responsible for this roadway thought likewise else they wouldn't have added the surfacing to it.
As I said in the first instance this is an unusual surfacing which I have not seen more widely used (thank goodness). If I had to describe the aggregate used in two words, I'd say it was 'shattered flints', any one of which might, once loose, be sharp enough to cut/puncture a bicycle tyre.
cheers