You are correct, and it doesn't even require a one block gap. Simply clipping the corner of a block can allow a player to clip through as well, which is how you

You are correct, and it doesn't even require a one block gap. Simply clipping the corner of a block can allow a player to clip through as well, which is how you access world -1. That particular glitch can be exploited at either the very top of the 1-2 pipe, or at the open edge of that same pipe that Mario would normally enter. Every video one can find about world -1 will exhibit this glitch. The article is really only technically correct, as those are glitches and certainly not intended, but in actual practice, yeah, it's not totally correct.