Except, you know, the rest of the world had the coronavirus before China even knew it was a real problem. It wasn't discovered by anyone until around December, but they think it originaged in November. Here in the US, our first confirmed a case was on January 20th, but the CDC has findings to suggest it was here at least 8 weeks earlier. That means we probably already had it by the time China knew what it was.
So let's assume China, some time in December, immediately recognized what they had, immediately went full lockdown and told everyone "YO WE HAVE A CODE RED, SATAN IS ON THE LOOSE."
What would that change about a virus already in at least 21 other countries? Probably already in at least 50, but we only can guess based on the date of their confirmed cases and findings. We know of a case where a guy returned from Spain in the beginning of February and found out he had coronavirus - that means he got it in mid-end January, which means Spain probably also had it since December.
Would it maybe spread slower if we had less international travel sooner? Probably true, but that wouldn't change that most countries didn't start going into lockdown until 60 days after we already knew COVID19 was worse than SARS.