If you take that as a rule, then everything presented as evidence in court is suspect from the get-go - you can’t believe anything at all that any sworn witness says because anyone can lie.
I’m not saying that testimony from friends who say that a murder suspect is a wonderful person are always or even mostly perjury. Those friends may sincerely believe the suspect is a wonderful person and be completley oblivious to his/her dark side. The point isn’t that they are all lies, but that they are completely irrelevant as evidence in a case.