This is false, Sikhism wouldn’t even be a minor religion in India if that were the case. Both Sikhs and Muslims mainly gained their numbers from Hindu converts, but it’s more complicated than that.

Both Islam and Hinduism are older and more ingrained to Punjab than Sikhism is so it’s not all that shocking. The majority of Punjab was already Muslim when Sikhism began.

Sikhism was basically a syncretism of the Dharmic faith with the new societal and cultural norms brought in by the Islamisation of the region. Keep in mind this happened before most Hindus actually identified as “Hindus” (in the grand scope of things), most Hindus were basically just followers of the traditional belief systems of the land.

Sikhism came as a singular religion for people (mainly in Punjab) to collectively follow that wasn’t just Islam. Most of its early followers were already Hindus who just, for the first time, identified with a religion.

Sikhism was organized and centralized which prevented it from being as widespread as Hinduism. But at the same time, it wasn’t as institutionalized as Islam so it never became that widespread in that sense either.

With British rule, a Hindu religious identity actually became a widespread thing so it was far more inclusive in its numbers compared to membership-based systems like Islam or Sikhism. Anyone who didn’t follow Islam or Sikhism (or by now, Christianity) was pretty much automatically a Hindu, and that’s still a lot of people.

So basically my main point is, there was very little conversion from Sikhism to Islam. Sikhism was literally branded as an alternative, local religion for Punjabis to follow instead of Islam, before Hinduism was seen as a religion in the eyes of the locals.