Islam means submission to god and sikhism too said that we should submit too god's will .
Here is the shabad of guru nanak about who is true muslim.
Sri Granth: Sri Guru Granth Sahib
ਸਲੋਕੁ ਮਃ ੧ ॥
सलोकु मः १ ॥
Salok mehlā 1.
Shalok, First Mehl:
ਮਿਹਰ ਮਸੀਤਿ ਸਿਦਕੁ ਮੁਸਲਾ ਹਕੁ ਹਲਾਲੁ ਕੁਰਾਣੁ ॥
मिहर मसीति सिदकु मुसला हकु हलालु कुराणु ॥
Mihar masīṯ siḏak muslā hak halāl kurāṇ.
Let mercy be your mosque, faith your prayer-mat, and honest living your Koran.
ਸਰਮ ਸੁੰਨਤਿ ਸੀਲੁ ਰੋਜਾ ਹੋਹੁ ਮੁਸਲਮਾਣੁ ॥
सरम सुंनति सीलु रोजा होहु मुसलमाणु ॥
Saram sunaṯ sīl rojā hohu musalmāṇ.
Make modesty your circumcision, and good conduct your fast. In this way, you shall be a true Muslim.
ਕਰਣੀ ਕਾਬਾ ਸਚੁ ਪੀਰੁ ਕਲਮਾ ਕਰਮ ਨਿਵਾਜ ॥
करणी काबा सचु पीरु कलमा करम निवाज ॥
Karṇī kābā sacẖ pīr kalmā karam nivāj.
Let good conduct be your Kaabaa, Truth your spiritual guide, and the karma of good deeds your prayer and chant.
ਤਸਬੀ ਸਾ ਤਿਸੁ ਭਾਵਸੀ ਨਾਨਕ ਰਖੈ ਲਾਜ ॥੧॥
तसबी सा तिसु भावसी नानक रखै लाज ॥१॥
Ŧasbī sā ṯis bẖāvsī Nānak rakẖai lāj. ||1||
Let your rosary be that which is pleasing to His Will. O Nanak, God shall preserve your honor. ||1||
so this is the defination of true muslim according to guru nanak.but if you ask any muslim scholar he will reject this defination.according to him a true muslim is one who goes to
hajj,do animal sacrifice,read namaz etc.so the defination of true muslim is different of guru granth sahib than the defination of true muslim of islam.
Btw hannah i just remembered that you already asked this question earlier and i too replied to you at that time.