I was considering this piece of Presidential trivia recently when I realized this seems to be the answer. I can't find any specific reference to this question online, but the rest of the early Presidents seem to have met in the 1770s, or 1769 for Washington and Jefferson, while the younger Adams was born in 1767. If my conclusion is right this might make for a fun trick question when discussing Presidential facts!
I think you are correct. The only other possibility I can think of is that Jefferson and Washington may have met socially some time before their joint service in the Virginia assembly (1769, as you said), but I can't seem to find any evidence for that. John Adams was geographically isolated in the north and the other early Virginian presidents (Madison, Monroe) were too young. Madison was 16 in 1767 while Monroe was 9.