The first definition of "endorse" contains the word "support." So, Johnson is wrong.
But you see what he -- and other vulnerable Republicans, including Sen. Kelly Ayotte of New Hampshire, who has tried similar verbal gymnastics when it comes to Trump -- are trying to do. They are trying to draw a bright line between endorse (passive, box-checking) and support (active, affirmative backing). Endorsing Trump is the obligatory thing a Republican has to do when it comes to Trump as the party's presidential nominee. But it doesn't mean the same thing as supporting Trump, which is the sort of thing that, say, Chris Christie is doing.
Yes, this is a classic example of trying to have your cake and eat it too. Trump presents a major problem for the likes of Johnson and Ayotte, who are trying to get reelected this fall in states that President Obama carried in 2008 and 2012. Trump's unpredictability as a candidate, coupled with his deeply negative poll numbers among a number of swing voting blocs, make him a dangerous figure for vulnerable incumbents. Totally abandon Trump and run the risk of the GOP base, who clearly chose him as the party's presidential nominee, abandoning you. Stick too closely to Trump and you could lose any chance of wooing independent and Democratic-leaning voters you need to win.
It's a lose-lose. Which is why people like Johnson are trying to make up some sort of difference between "endorse" and "support." But it won't work. It is a distinction without a difference, and one that comes from a transparently political place.
You can't hug someone as you are running away from them. Trying to pull off that maneuver will leave you looking dumb -- and accomplish nothing. Sorry, Sen. Johnson.