It's hard to say if WordPress is the best for SEO without extensively testing all platforms claiming the same.
No one has done a real world test drive ranking competing websites on multiple platforms for the same keywords and sharing the results...
Anyone saying otherwise is just click-baiting you
A better question on the other hand would be:
That I can positively answer "yes, it will" to it.
WordPress will give more control and will help you better and easily implement a couple of on-page SEO related features that will require an extensive/expensive work if done on other platforms.
Here is a couple of them:
When you have a ranking website on another platform, making a switch to WordPress can be tricky and risky if not handled properly.
Attention to details is vital.
For example, you pick the wrong permalink while omitting redirects and you risk losing it all.
That kind of focus and attention doesn't usually come like Fiverr cheap
The more pages you have, the more demanding the task will be to make sure to keep at least the ranking intact.
Depends on your current state and how the competition is ranking.
I would say there will be an improvement for sure IF everything is done properly. As I said, WordPress has the tools to help with SEO. But using them properly is up to the service provider's capabilities.
Hard to say without analyzing how you compare to competition.