That was my source too. However after further reading I think it may not be correct.
http://www.academusedition.cz/en/title/jan-zach-requiem suggests it was composed in the 1730s in Prague (It also refers to Zelenka). The description at Amazon of the cd I referred to, which seems to be copied from the cd booklet, also refers to the 1730s. The music is also more baroque than classical (as his later music is referred to). 1730s may also be more likely since at that time he was working in several churches in Prague, while in the 1760 he was more or less jobless.
In any event the requiem is beautiful. I also recommend his other works.