Yes, you are correct. "To strain themselves to understand" means "to work hard" or "exert an effort." An excuse that "strains credulity" is a metaphor. "Credulity" is the ability to believe. If someone says "I was late because of an accident on Route 128 that caused a traffic jam," that is easy to believe because it happens often. If someone says "I was late because a small meteorite struck my tire and caused a flat tire," we say that is "hard to believe." Therefore, it figuratively requires work, or takes an effort, to believe it. You might answer "I can't believe that story no matter how hard I try. It doesn't just strain my credulity, it stretches it past the breaking point."

Yes strain here mean "make an effort" it is implied that it is difficult or them to understand