The teachings of Baba Nanak and the Quran fit so so well – doesn’t this show aht Guru accepted Islam as his religion?

Not true. Guru Ji’s Bani (revelation) clearly show that His only Guru was God not a human being. Also the word “Guru” is not used by Muslims at all. Guru Ji taught that everyone is equal including women which directly conflicts with the teaching of Islam. Guru Ji taught that there are millions of heavens, hells, solar systems, galaxies, skies and earths but Muslims believe that there are only seven layers of sky up and down totalling fourteen. Guru Ji rejected the idea of priesthood, fasting and many other rituals practised by Muslims. Therefore, Islam could not have been Guru Ji’s religion because he openly rejected its backward teaching.

Muslims give some quotes from Bhai Bala Ji’s Janam Sakhi to prove that Guru Nanak Dev Ji believed in Islam. This Janam Sakhi is creation of 19th century and has been rejected by the Sikh scholars. Guru Ji’s writing clearly shows that He never accepted Islam. There is nothing of higher status than Guru Ji’s Bani itself. You can read yourself what Guru Ji thought of “real Kalima” and “Hajj”.

ਰੋਜਾ ਧਰੈ ਨਿਵਾਜ ਗੁਜਾਰੈ ਕਲਮਾ ਭਿਸਤਿ ਨ ਹੋਈ ॥
ਸਤਰਿ ਕਾਬਾ ਘਟ ਹੀ ਭੀਤਰਿ ਜੇ ਕਰਿ ਜਾਨੈ ਕੋਈ ॥੨॥
“Keeping your fasts, reciting your prayers, and reading the Kalma, the Islamic creed, shall not take you to paradise. The Temple of Mecca is hidden within your mind, if you only knew it. ||2||”
(Siri Guru Granth Sahib Ji, Ang 480)

Guru Ji rejected Islamic beliefs and practices. Another important point to be made is that many beliefs of all major religions will be similar and in tune with one another. For instance, monotheism or the belief that there exists only one God; because Christians believe it and Muslims believe it, it does not make Christians Muslim…nor does it make Sikhs Muslims.