The context meant that when Paul said that they didn't need circumcision, he was not trying to make an implicit argument that people don't need baptism, because he was arguing in the context of a real argument in the Church over whether gentiles needed circumcision. Paul's point was that gentiles don't need circumcision to get saved like the Jewish Christians got circumcised. He wasn't getting into the issue that Noah probably wasn't circumcised. He was saying that uncircumcised Gentile Christians got saved like Jewish Christians got saved.
Acts 15 begins by saying:
"And certain men which came down from Judaea taught the brethren, and said, Except ye be circumcised after the manner of Moses, ye cannot be saved."
That was the issue under debate, not whether people should belong to the Christian community.
The next verse says:
"When therefore Paul and Barnabas had no small dissension and disputation with them, they determined that Paul and Barnabas, and certain other of them, should go up to Jerusalem unto the apostles and elders about this question."
They decided to go to the elders to address the problem, instead of just having everyone decide it on their own.
Then in verses 8 and 9, after they consider the issue, Peter says that they have the same salvation:
"And God, which knoweth the hearts, bare them witness, giving them the Holy Ghost, even as he did unto us; And put no difference between us and them, purifying their hearts by faith."
I suggest reading the chapter:
www.biblegateway.com/passage/%3fsearch=Acts%2b15&version=KJV