Being curious myself, I have googled both the term and Hernandez (who declares he is one) and could not find a definition or explanation. My conclusion is that this "title" is simply one Hernandez invented for himself and may be explained as follows.
Millennials were born between 1981 and 1996.
A 2019 article titled "In U.S., Decline of Christianity Continues at Rapid Pace" says "Data from Pew [Research Center] between 2020 and 2021 reported that nearly half of millennials surveyed (49%) described themselves as Christians, the most of all religions, another 10% identify with non-Christian faiths and four in 10 now identify as religious 'nones.'
I'm guessing that Hernandez, who is a Baptist apologist, sees himself as a specialist in targeting millennials in order to get them "saved" such as Raf's second post about the conversion of the atheist Daniel Nieto shows. Why would he want to do this?
One reason might be that getting 28-43 year olds converted increases the probability that their children will be brought up in religious homes and as a result, reverse the increase of "religiously unaffiliated" which has consecutively occurred over the past 5 generations.