ADDING I think so too duinnsleibhe
The wonderful people who compiled the BISA obviously had access to all the records there were.
The way the records were linked to establish the individual identities is amazing.
My speculative guess is that the David THOMAS was admitted to either Hospital or Asylum in 1853 and he is distinguished from others of the name by his ship of arrival on that admission record. Also his marital status was possibly recorded.
Thus his entry in BISA states, married by 1853.
Possibly also the family of his wife (his only family in the colony) did not place a newspaper death notice ( if he did in fact die in care), because it did not suit their perceived social position

Sue