from Setext heading spec,
…The lines of text must be such that, were they not followed by the setext heading underline, they would be interpreted as a paragraph: they cannot be interpretable as a code fence, ATX heading, block quote, thematic break, list item, or HTML block.
My Question : Why they cannot be interpretable as other blocks?
Why the lines of text which aren’t followed by the setext heading underline cannot be interpretable as a code fence or etc? I understood above like this way.
So, for example,
# abc
This line of text aren’t followed by the setext heading underline, but we always interpret it as a ATX Heading.