question on mool mantarr

I was reading some old posts from tapoban forum and found 1 post in which Bhai Kulbir Singh mentioned that in sikhi there is only 1 mantra and one can call it beej mantar or mool mantar or gurmantar.
Just want to know whether it is trure. Satguru has given no other mantar to sikhs other than gurmantar ?

ਅਵਰੁ ਨ ਅਉਖਧੁ ਤੰਤ ਨ ਮੰਤਾ ॥
ਹਰਿ ਹਰਿ ਸਿਮਰਣੁ ਕਿਲਵਿਖ ਹੰਤਾ ॥

ਹਰਿ ਹਰਿ refers to Gurmat Naam ( Gurmantar) and there is
no other rememdy. In addition, BHai Daya Singh Ji rehatnamey mentions not
to recite any other mantra.

BeejMantar refers to planted mantar- meaning mantar which is planted
during initiation into Gurmat ( WaaheGuru)

Mool mantar refers to original/first mantra which came from WaaheGuru ( WaaheGurus DargaH). All other mantras ( names) have origins in polytheism. For example, Allah refers to moon God of polytheist; Gopal refers to cow God. Har, BhagwAN, kRISHAN, ALL REFER TO KRISHNA, Raam, orignally referred to Ram Chander, All previous avtars were in Haumi and only gave mantra of their own name while Guru Sahib only gave original mantar ( WaaheGuru). So how can Guru Sahib given any other mantar.