“The constitution cannot be amended by congress passing a bill and the president signing it.”
No one posting here said that it could be amended in that manner.
Saying that congress could create new "natural born citizens" by passing a bill and getting the president to sign it is the same thing as saying the constitution can be amended by statute.
Congress has an acknowledged power of naturalization, but naturalized and natural born are distinctly different. Congress can't make "natural born", they only have the power to make "naturalized."