I completely agree Blumenthal lied about his service. He said, "when we returned", for example. In the most charitable case, he was identifying with other soldiers (reserve plus active combat), and in the least charitable, just wanted the credit for being in Vietnam when he was in the Reserves. 

But Trump isn't happy to say that. Blumenthal made a mistake and lied about "returning", but I see no record of "his stories of battles and conquests". Trump knows that Blumenthals mistakes are subtle, so he lied to make the error more egregious. Those words he tweeted were a lie, wouldn't you agree?