At first glance it may seem obvious to disagree with the statement, given that it was God's initial covenant with Abraham which began the divine relationship with Israel, and as such without it the Mosaic covenant would most likley not have existed at all. However, we must remember that although the Abrahamic instigated the beginnings of a monotheistic worship, it was the Mosaic that formed the Israelite community. The ten commandments dictating orders for divine worship as well as social regulations which allowed the bringing together of the Israelite people as one, having previously had no experience of living as an independantly governed community, leading either nomadic lifestyles or being held as slaves in Egypt.
Also, we may argue that the very occurance of the Mosaic covenant proves it to be more important, as if the Israelites had not strayed away from God (for example by performing Idolitary worship to Baal) there would be no need to re-affirm the divine-human relationship and set out regulations for living and worship. We may argue that it is the lack of clear direction from God seen within the Abrahamic covenant which allows for the people to be easily lead astray, whereas the Mosaic binds them firmly as a community.