Since nobody has really provided the answer plain and simple:
A third party keyboard can not see what you type with another keyboard, unless you switch to that keyboard. If you're concerned about having the keyboard installed but only don't want it to see what you're typing in specific instances, then you don't need to worry because it can't access your text unless you're using it.
From a technical standpoint, keyboards implement a UITextDocumentProxy. Per apple's description, "A text document proxy provides textual context to a custom keyboard".
https://developer.apple.com/library...ITextDocumentProxy/documentContextBeforeInput
A text document proxy can do 5 things:
1. Get the text before or after the typing 'cursor' in order to determine context. They don't really specify how much text it gives, but it's only going to provide data from the text field that has focus, not any others.
2. Adjust the position of the typing cursor.
3. Insert text
4. Delete text
5. Check if the text field is empty.
These activities can only be accomplished when a custom keyboard is running, so if you're using the apple keyboard SwiftKey won't have access to any data that you type. If you're in a text field and switch to SwiftKey, they can request the text already there for context purposes.