“Judaism was always open to converts. There is a difference between becoming a convert and becoming a second-class “graft”
Nothing about conversion to Judaism or second class grafts. The statement is clear that Gentiles will be on the same level in the kingdom as the Patriarchs. In fact Jesus says “but the children of the kingdom shall be cast out into outer darkness: there shall be weeping and gnashing of teeth”. Those that are trusting in circumcision and the dietry laws will be excluded while those that have faith, whether Jew or Gentile, will be included.
Obviously Christ tought some Jews were not "true" believers and was astonished [sic], marvelled, or amazed by the faith of the (Roman) Centurion (in a Jewish messiah)! That's the last thing one would expect from a Roman officer. The way Mat 8:10 is worded, it seems Jesus was caught by surprise.
That leads me to believe that the writer of those verses did not think Jesus would have known, unless he was writing for effect.