Ambarsaria ji,
I hope this answers your question on why they are using the word "Waheguru". It doesn't just mean wonderful teacher, it is also referring to the teacher inside of you and God. The later being the same in non-dualism, that is, the teacher inside you is God. So no they are not calling out Sri Guru Granth Sahib Ji (although if they were that would not change much).
Look how Bhai Gurdas ji poetically describes Waheguru mantr. Though one need not use it as it is just a word one uses to remember and repeatedly identify with the (inner) God.
੪੯ : ਵਾਹਿਗੁਰੂ ਮੰਤ੍ਰ
Waheguru Mantra
ਸਤਿਜੁਗ ਸਤਿਗੁਰ ਵਾਸਦੇਵ ਵਵਾ ਵਿਸਨਾ ਨਾਮੁ ਜਪਾਵੈ।
Satijougi Satigur Vaasadayv Vavaa Visanaa Naamu Japaavai.
सतिजुगि सतिगुर वासदेव ववा विसना नामु जपावै ।
In Satyug, Visnu in the form of Vasudev is said to have incarnated and ‘V’ Of Vahiguru reminds of Visnu.
ਦੁਆਪੁਰਿ ਸਤਿਗੁਰ ਹਰੀ ਕ੍ਰਿਸਨ ਹਾਹਾ ਹਰਿ ਹਰਿ ਨਾਮੁ ਜਪਾਵੈ।
Duaapari Satigur Haree Krisan Haahaa Hari Hari Naamu Japaavai.
दुआपरि सतिगुर हरी क्रिसन हाहा हरि हरि नामु जपावै ।
The true Guru of dvapar is said to be Harikrsna and ‘H’ of Vahiguru reminds of Hari.
ਤ੍ਰੇਤੇ ਸਤਿਗੁਰ ਰਾਮ ਜੀ ਰਾਰਾ ਰਾਮ ਜਪੇ ਸੁਖੁ ਪਾਵੈ।
Taytay Satigur Raam Jee Raaraa Raam Japay Soukhu Paavai.
तेते सतिगुर राम जी रारा राम जपे सुखु पावै ।
In the the treta was Ram and ‘R’ of Vahiguru tells that rembering Ram will produce joy and happiness.
ਕਲਿਜੁਗਿ ਨਾਨਕ ਗੁਰ ਗੋਵਿੰਦ ਗਗਾ ਗੋਵਿੰਦ ਨਾਮੁ ਅਲਾਵੈ*।
Kalijougi Naanak Gur Gobind Gagaa Gobind Naamu Alaavai.
कलिजुगि नानक गुर गोबिंद गगा गोबिंद नामु अलावै ।
In kalijug, Gobind is in the form of Guru Nanak and ‘G’ of Vahiguru gets Govind recited.
ਚਾਰੇ ਜਾਗੇ ਚਹੁ ਜੁਗੀ ਪੰਚਾਇਣ ਵਿਚਿ ਜਾਇ ਸਮਾਵੈ।
Chaaray Jaagay Chahu Jougee Panchaain Vichi Jaai Samaavai.
चारे जागे चहु जुगी पंचाइण विचि जाइ समावै ।
The recitations o f all the four ages subsume in Panchayan i.e. in the soul of the common man.
ਚਾਰੋ ਅਛਰ ਇਕੁ ਕਰਿ ਵਾਹਿਗੁਰੂ ਜਪੁ ਮੰਤ੍ਰ ਜਪਾਵੈ*।
Chaaro Achhar Iku Kari Vaahaguroo Japu Mantr Japaavai.
चारो अछर इकु करि वाहगुरू जपु मंत्र जपावै ।
When joining four letters Vahiguru is remembered,
ਜਹਾਂ ਤੇ ਉਪਜਿਆ ਫਿਰਿ ਤਹਾਂ ਸਮਾਵੈ ॥੪੯॥੧॥
Jahaa Tay Oupajiaa Dhiri Tahaa Samaavai ॥49॥1॥
जहा ते उपजिआ फिरि तहा समावै ॥४९॥१॥
The jiv merges again in its origin.
7 ਵਾਰਾਂ ਭਾਈ ਗੁਰਦਾਸ : ਵਾਰ ੧ ਪਉੜੀ ੪੯ ਪੰ. ੭
Bhagat Singh ji,
Guru Fateh.
The fact of the matter is that poetry of Bhai Gurdas did not pass the benchmark set by of our visionary Gurus. This is the reason it was rejected and hence not added to SGGS, our only Guru. This was our Gurus' decision which should be respected.
So, to compare Bhai Gurdas' poetry with SGGS, is not only undermining our Gurus' decision but also undermining SGGS, our only Guru. And both are not wise things to do as Sikhs.
Thanks & regards
Tejwant Singh