Comment by glenstein

Seconded. Sometimes when someone says XYZ was likely used it's because they've read something from a credible source, or maybe are a subject matter expert, or have grasped some other similarly solid chain of evidence.

But sometimes, they mean "likely" in the more colloquial sense of a guesstimation, which can range anywhere from informed guess to low effort fan-fiction. I default toward the latter unless otherwise specified.