Vishwamitra is a Brahmarshi because of the boon

शिवसुत उवाच |

 

Son of Shiva, Kartikeya said (to Vishwamitra)

 

विश्वामित्र वरान्मे त्वं ब्रह्मर्षिर्नात्र संशयः |

वसिष्ठाद्याश्च नित्य त्वां प्रशंसिष्यन्ति चादरात् || 12 ||

 

Vishwamitra, (because of) my boon (to you), no doubt here that (you are a) Brahmarshi (a Sage of the highest level), and Vasishtha, etc. will everyday/always praise you and (that too) with respect.