The couple linked appear to be duplicates of https://www.wikitree.com/wiki/Maartens-80 and his wife. (Have proposed merges). Maartens-80 was baptised in Capetown, son of an emigrant from what is now North Western Germany and a woman born in the Netherlands . His wife https://www.wikitree.com/wiki/Beck-13746 also has European parents.
The profile of John Martin convict says he was black and possibly an American seaman (secondary sources on the web support this e g. Martin, John (c. 1757–1837)', People Australia, National Centre of Biography, Australian National University, https://peopleaustralia.anu.edu.au/biography/martin-john-30316/text37596, accessed 29 January 2024.
Not definitive evidence but I think it is most unlikely that John Martin would have been the grandson of emigrants from Northern Europe to the Cape of Good Hope.
