About why it's supposedly Hindustani, there's an early 19th century song called "I have come from a happy land, the celebrated dancing girls song, arranged to a Hindustani air by R.A. Smith". There's no date on the manuscript, but Smith lived 1780-1829. Stick the title into Google and various manuscripts come up.

The air, with that name and arranged for union pipes with regulator accompaniment, is in Robert Millar's manuscript of union and Northumbrian smallpipe tunes from 1830, Montrose, Scotland, which is at "Ross Anderson's Music Page". RA Smith, who lived in Edinburgh, published it in 1827 including two other tunes he claimed were Hindustani. Maybe they were, but who knows.

Frank Kidson wrote about it here.

https://tradsong.org/wp-content/uploads/2016/09/Secular-Tunes-II.pdf

The subject of the words varied but the idea of a far away land - India, heaven, where old soldiers go to - stayed.