Why was Will Salas so sure that Timekeepers would not jump through the window?

Because most of the Time Keepers came from the upper class/more wealthy parts. As such, they aren't used to having to take risks and shortcuts like those of lower class. This is a part of what separates Raymond from his fellow Time Keepers; he knows the streets and is used to taking risks and short cuts to get what he wants because he grew up poor.

In the established universe, those of higher class have the luxury of taking their time to do things. They don't have to run everywhere just to stay alive. As such, unless it's for something fun they have no real reason to do dangerous things, such as jumping out of windows.

Will wasn't so sure Raymond wouldn't jump, but he took a risk to stick around and watch knowing that if Raymond attempted it, he'd still have time to get away. It was a calculated risk, because Will wanted to know how close the Timekeepers were to him. If he just ran, he'd never know if they jumped or not, and he'd have no idea which escape route was more likely to work.