The idea is that to get results, you need some combination of a lot of data, or a big impact from the changes your testing. If you just change the color of the signup button, it probably won't have a major impact on the conversion rate, so you'll need a lot more data to reach a conclusion. But if you test a completely new landing page, it might have a better chance of being meaningfully different (better or worse, who knows until you test?) and so you wouldn't need as many visitors to get a result.
The idea is that to get results, you need some combination of a lot of data, or a big impact from the changes your testing. If you just change the color of the signup button, it probably won't have a major impact on the conversion rate, so you'll need a lot more data to reach a conclusion. But if you test a completely new landing page, it might have a better chance of being meaningfully different (better or worse, who knows until you test?) and so you wouldn't need as many visitors to get a result.