There are different ways to affect the text size in a phone.
In iOS, Settings > Display & Brightness > Text Size > Drag the slider for text size (A).
In Android, Settings > Display > Screen Zoom > Font Size and Style > Drag the slider for font size (B).
In iOS, Settings > Display & Brightness > Display Zoom there is an option for Larger Text (C).
The get method of the Capacitor Text Zoom Plugin returns the current text size. For Android this relates to (B). For iOS this relates to document.body.style.webkitTextSizeAdjust which is an iOS specific property that affects the text size of all text.
The getPreferred method of the plugin returns the current text size. For Android this relates to (B) and for iOS this relates to (A).
The set method will affect the size of all text in your app (document.body.style.webkitTextSizeAdjust) and for Android it will also do the same. It does not change the setting of your device.