It's interesting but it doesn't mean anything in the grand scheme of things except for Zverev holding a lot of belief in himself. The German has been a good clay performer over the years but he played particularly good last year on clay. It's not impossible that he would have won that match because he was pretty close and Nadal needed to pull off some spectacular rallies in order to win the opening set.
He spoke about that in Monte Carlo explaining that he doesnt' see it as an arrogant view:
"For some reason I did. I don't want to sound arrogant. Rafa is obviously the best player to ever play on that surface so you never know what happens. You never know what happens in that match as well. If I don't get injured, of course I [could have lost] that match. Of course, he can go on to win his 14th Grand Slam."
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"But I felt like I was playing my best tennis that I've ever played on that surface. So for some reason, I did feel like I could at least compete with him, which I was doing. The outcome of the match obviously, always depends on little factors as well. But I did feel like that could have been the week for me."
Asked about his R1 match in Monaco, Zverev said:
"I have to be ready. I think against him especially you have to be ready to play at your best and you also have to be physically ready to run. You never know what you're quite going to get. So at the end of the day, I'm just looking forward to being back and playing and hopefully playing some good matches here."