Ok so if I typed in a credit card number using Apple's keyboard than downloaded SwiftKey, it wouldn't have that info. Right?
It throws me when it says "including things you have previously typed with this keyboard". Does that mean Apple's keyboard or SwiftKey's?
SwiftKey does learn what you type and that includes numbers. When I was moving and I was typing my house number a lot, SwiftKey would predict my house number after entering the first digit. Any input field that SwiftKey can see, it can remember in order to better predict and correct what you type. If you can enter your credit card number in any input field that is visible, then SwiftKey will learn it and suggest it later if it finds you are constantly typing that number. This is how SwiftKey works and why it is considered one of the best keyboards when it comes to text prediction and auto correction.
SwiftKey does not offer suggestions whenever I type into a password field where the characters you type are hidden. On my Android phone, SwiftKey never makes suggestions or corrections.
If you use multiple devices, SwiftKey has a cloud feature. What it learns from one device it can apply to other devices. This feature works across platforms as well. I use SwiftKey on my Android phone. When I installed it on my iPad Mini, I activated the feature so it can take advantage of what it learned from my typing on my Android phone. As my typing style and words I use don't change across devices, SwiftKey can make accurate predictions and corrections immediately without having to relearn my typing style on my iPad Mini.
Now whether or not someone who works for SwiftKey can actually go in and look at what you type, that is more to due with the security policy within the company. I think this is possibly what you may be concerned about. That I don't know. I do not recall seeing SwiftKey ever many any announcement about their security policies.
It all comes down to a matter of trust. SwiftKey is a well trusted app on Android. It is one of the most popular third party keyboards on that platform. I do not recall hearing anything that suggests any issue with their security practices.