The title seems to give it an out by not having to admit all it's stories are true. The skeptic in me thinks that some of the stories are knowingly false. Is there any info out there that says either way?
According to this, it seems as though his intention was to only put forth true tales.
Ripley claimed to be able to "prove every statement he made" because he worked with professional fact researcher Norbert Pearlroth, who assembled Believe it or Not!'s array of odd facts and also verified the small-town claims submitted by readers. Pearlroth spent 52 years as the feature's researcher, finding and verifying unusual facts for Ripley; after Ripley's death, for the King Features syndicate editors who took over management of the Believe it or Not! panel.
Unfortunately that's unsourced. This article mentions these things:
Despite Ripley’s avowal that everything in his cartoon was absolutely true, many readers simply refused to believe him, and they wrote letters, sometimes thousands each day.
He started by making a salesman’s pitch to his new readership, promising that his Believe It or Nots “are all true,” and if any readers questioned the facts, he’d “prove the truth” to any doubters.
He also hired someone specifically for research/verification:
When it came to cartoons featuring some math, science, or history puzzler, Ripley increasingly relied on the help of a silent partner, Norbert Pearlroth, a former banker and accomplished linguist with a near photographic memory. Ripley had hired Pearlroth in 1923 as a part-time research assistant.
So, it appears his intent was to just present strange stories that didn't sound true, but that he could back up.