I think the wording of your question means you realise that the image of the family tree of Stewart Buyers Chrystall is not a reliable source, and that what it shows should be verified from better sources. That means that the birth (1689) and death (1766 - not 1776 as in the question) dates shown for William Moir should be regarded with caution. As far as the death date is concerned, one would really need to find a record which shows a William Moir who died on 4 April 1766 together with evidence that this relates to the William Moir who married Jean Jamieson.
A quick search on the web shows that there were quite a lot of William Moirs in Aberdeenshire in this period. So great care is called for. Even just looking at records for Dyce, Aberdeenshire, according to the Scotland Marriages database on Familysearch, there is a William Moir who married Jane Frost at Dyce on 9 December 1725, and there are at least two baptism records for their children (I did not look for more) - see https://www.familysearch.org/ark:/61903/1:1:XYX7-Y3N and https://www.familysearch.org/ark:/61903/1:1:XYX4-HMJ: this is a different William Moir.
[edited for typos]
