Two things, arguably, need be kept in mind.
I.
AA. Sikhism was initiated by Guru Nanak and it was a part of the (broad) Bhakti (and Sufi) traditions.
It preached universal love and Nanak made deliberate attempt to coalesce elements of what's referred to as Hinduism and Islam.
There's a story common to both Nanak and Kabir - another leading exponent of the Bhakti (and Sufi) traditions.
When the Master died, the disciples - Hindus and Muslims, kicked up a fight as they both wanted to cremate the departed as per their own traditions.
In the process, someone suddenly removed the sheet covering the dead; and lo and behold: there was no body - only flowers.
The flowers were divided between the two feuding camps.
BB. It's in response to bloody and brutal persecutions carried out by the ruler of the day - Aurangzeb, Guru Gobind Singh completely overhauled Sikhism and turned it into a militarised religion - every male carrying arms and ready to fight.
It's also said that the eldest son from the Hindu families would be baptised as a Sikh.
In course of time, it acquired its very own distinctive identity.
II. Which would come to be known as "Hinduism" - using a convenient tag provided by incoming outsiders from across the Sindhu river, was, and to an extent still is, a broad basket of loosely connected divergent traditions.
In a way, all Indic traditions were parts of this broad basket.
But, things evolved over time - both spontaneously and by design.
Many coalesced closer and a few acquired distinct independent identity.
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Such a nuanced and objective view would, for good reasons, be despised by communalists - of all hues, and, also, the "secularists" - at least, too many of them.
"History" is all too often looked upon as a weapon in the fight for the present and not a field for dispassionate objective study.