On what grounds do you say that Trump's policy wasn't based on evidence?

The evidence* is that providing transgender surgery and hormones would cost the US military between $2.4 million and $8.4 million per year, in addition to the time lost. That's to cover the estimated 29-129 service members who would seek the treatment yearly. It's rational to ask, is that additional cost worth the small number of soldiers gained from it? While that matter is being studied it's reasonable to put a hold on transgender people joining the US military. And that's what Trump did. 

It's also reasonable to assume that, as did 18 other countries including Canada, the US military will conclude that allowing transgendered people to serve does not seriously impact the force's ability to function.

* The Implications of Allowing Transgender Personnel to Serve Openly in the U.S. Military - Rand Corporation

https://www.rand.org/pubs/research_briefs/RB9909.html