I hear this every time the subject of stealing, or borrowing, music from another source comes up, and I blatantly disagree with the intent. What this statement says is that we're so limited by the western major scale that eventually songs, or song parts, will be repeated. With a 60k songs uploaded to Spotify every day, you can see the logic here, but, consider all the current songs you know using the same progression, or the same chords. "The Passenger" by Iggy Pop, "I'm Yours" by Jason Mraz and "The Scientist" by Coldplay are all the same progression, are these the same song? Same part? Different tempos alone make the progression hit differently, not to mention intonation and style. I don't know much about probability, but I know if you give 25 guitar players three chords they will all come back with a distinctly different part, even in the same order. Go write something with G-C-D, I'm sure you won't have two of the same parts.
The interval is important. This is commonly known as the number of notes between one and the next, between G and D for example. But time is a factor too. Do you move quickly from G to C and then slower to D, or vice versa? And this is only considering one part; a verse, a chorus etc. Adding other parts contributes to a totally different song with a totally different feel. If you write an existing song again you were either trying to do it or binge listened to it and unintentionally wrote it, which at that point as Tom Petty said, "you don't usually let it get out the door".
I think when you're trying too hard to make a hit, really pushing and forcing creativity, your brain reaches for something it knows is popular, and something you like as well. So you unintentionally conjure up a good tune in your subconscious and as you're working through chords you'll write that song again. Conversely, if you relax, don't focus on writing something people will like or want and listen to what your soul is saying, you may write something inspired by music you love, but it's hard to plain steal something.
I tried to steal a riff once, a bassline actually. It's the song "rings" by Pinegrove. The baseline is a distinctive groove that got into my head. So I tried to play it myself without knowing the notes. I figured I could "borrow" the groove and substitute different notes. Well I couldn't figure it out so a quick internet search brought me the notes and I then tried to play it in a different key, or change it slightly. Nothing worked, fittingly. You shouldn't try to appropriate people's work. Evan gets to have that one, as well he should, and I need to do my work and write my own "I wish I wrote that" tune.