As to whether Abigail and Liberty were texting with a boy and had a secret plan to meet him in the park, I've never thought that was likely.
Going to the park on February 13, 2017 was a spontaneous event. Abby and Libby had few specific plans for the time they were about to spent together. On the evening of February 12th, when Abby arrived for the sleepover, she brought paint with her and Abby and Libby spent the evening painting a picture together that they titled "Chocolate". They were up half the night talking.
They slept in and make pancakes for breakfast the next morning. The girls asked what they could do, so Libby's grandma asked if they would like to do some filing to earn money that they could use to shop. They did that for a while and were not on their phones while filing.
While the girls were filing, Libby's older sister, Kelsi, said she was going to a friend's house before she had to go to work at 4pm. Libby jumped up and asked if Kelsi could take the girls to the trails, which she did, with the girls being dropped off between 1 and 1:30pm. It was a nice day and the decision to go to the park was spontaneous.
There are many reasons why Libby may have started photographing/recording the man. He may have said something scary before he said "down the hill" that alarmed the girls. Many websleuthers have commented that they think the bridge guy may have been carrying a weapon that the grainy picture doesn't make clear. Maybe Abby and Libby could see a gun or a hammer or who knows what that scared them.
I've always thought this could have been a crime of opportunity by someone who knew students were off school that day. Look at the apparent layers of clothes on the suspect versus how Libby and Abby are dressed. I checked the temperatures in Delphi, Indiana on February 13, 2017 when Abby and Libby were in the park. The temperature was about 43-44 degrees F and sunny. Maybe the perpetrator had been dressed warmly and hanging out all day looking for potential victims.