Im not sure but in this pauri I think rehat without sihari refers to Gurmat Maryada specifically the maryada of taking Amrit, and taking care of Kes according to Gurmat while Rehat with sihari the meaning means "without". My simple understanding of this pauri is:
ਬੇਦ ਰਹਿਤ ਜਿਮ ਦਿਜ ਨ ਸੁਹਾਵਤ। ਸੀਲ ਰਹਿਤ ਕੁਲ ਨਾਰ ਨਾ ਭਾਵਤ॥
Without the customs of the Vedas the ritual birth is not beautiful. Without good conduct ( patience) the women is not pleasing to the family
ਬੇਗ ਰਹਿਤ ਜਿਮ ਬਾਜੀ ਹੀਨ। ਰਹਤ ਬਿਨਾ ਤਿਮ ਕੇਸ ਮਲੀਨ॥
Without force the drum produces no sound. Without Sikh Rehat the Kes becomes dirty.
ਜਿਉ ਕਾਹੂੰ ਲੈ ਗਧਾ ਨਵ੍ਹਾਇਓ। ਫੂਲ ਮਾਲ ਤਿਹ ਮੂੰਡ ਧਰਾਇਓ॥
Some fools bathe and then place a garland of flowers over their head ( Hindu ritual)
ਤਿਹਕਰ ਤਾਕੀ ਸੋਭਾ ਨਾਹੀ। ਹਾਸੀ ਜੋਗ ਹੋਇ ਜਗ ਮਾਹੀ॥
But in this world these people gain no glory/blessings instead people laugh at them.
ਤਿਮ ਕੁਰਹਤੀਏ ਕੇਸ ਰਖਾਏ। ਕਹੋ ਸੋ ਕੈਸੇ ਸੋਭਾ ਪਾਏ॥
One who keeps rehat but acts contrary to Gurmat then how can such people gain glory ( reach SACH Khand)
ਰਹਤਵਾਨ ਜਗ ਸਿੰਘ ਜੇ ਕੋਈ। ਗੁਰ ਕੇ ਲੋਕ ਬਸੇਂਗੇ ਤੇਈ।
If there is a Sikh in this world who keep rehat then they reside in sach khand and gain glory.
Im sure there are many mistakes I have made but this is just my simple interpretation.
I think the gist of this pauri is to warn GurSikhs to keep rehat of their kes according to Gurmat instead of keeping rehat according to other faiths.For example, we know of some " Sikhs" who say they wont wash their kes on Tuesday, because then their husband will be cursed. Lol this is a Hindu belief. Gursikh washes his kesh everyday. In Prem Sumarg Rehatnama there has been some tampering and I do remember a ritual about bathing and wearing flowers. I don't know if this comes from Hinduism.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥