Also, even the proposed effect is modest:

> But early friendship bonds played an even bigger part than maternal relationships in the ways people navigated adult friendships and romantic partnerships, accounting for 4 percent of the variance in adults’ romantic partner- and best friend-specific attachment anxiety, and 10 to 11 percent in their partner- and best friend-specific avoidance.

Just slightly less modest that analogous parental predictors, according to their claims.