In part, I expect they are saying this to appease locals who are concerned about traffic levels. They want to go "we're investing in something new, it's to stop us losing guests, not to get us more guests". The logic being that people are coming but not coming back, and a new ride is needed to get them to come back.
Another reason to say this is simply because it's not realistic. In 2019 (the last year we can accurately compare with), the park were over 500,000 visitors short of that number. There's very little chance that any ride would bring that extra number of people in.
I mean, the park was very busy every peak day. There were no significant problems, but I think it should be remembered that operating at that level continuously then requires more staff, more maintenance, etc. I'm not saying the park were struggling to cope then, but I think given the current set up of the park, they would struggle if they did get half a million extra visitors in one year. Any park at a similar scale to Thorpe would too.