Those characters are not the result of any decoding process. They are simply a way of showing signal strength vs. frequency and direction (actually, time due to the Earth’s rotation).
They show that the Earth’s rotation caused the antenna to scan some source of considerable intensity and narrow bandwidth (< 10 KHz) in a way that no terrestrial or solar source could be expected to duplicate.
But as the article says, it could have been a reflection off of a satellite or space junk, which might be able to mimic they dynamics of an extraterrestrail signal source, as seen by the project’s antenna.
Everyone should “remember the pulsar” (LGM-1, 1967) which we thought was an intelligent signal.