SwiftKey does not learn anything from fields marked as password fields, nor does it remember long numbers such as credit card numbers.
Furthermore, we take privacy very seriously and no personal data that you type leaves your device unless you specifically opt-in to services that may access your data to provide benefits - such as backup & sync or personalization. All data is encrypted before it leaves your mobile device in such situations. There is more information available about data security here.
The warning message from Google that says SwiftKey may be able to collect ‘all the text you type, including personal data like passwords and credit card numbers’ (pictured below) is a part of the Android operating system that appears when any third party keyboard is enabled.
If you choose to personalize SwiftKey with popular services such as Facebook or Gmail, we never store your username or passwords. Instead, your credentials are handled completely by each third party service, and SwiftKey never has access to those credentials.
If this answers your question and you have not yet completed the SwiftKey installation, please click on the SwiftKey app on your phone to continue enabling SwiftKey.