The link provided by user1879822 was actually the most useful one for me: https://build.phonegap.com/blog/access-tags
To summarize, PhoneGap has a whitelist of allowed URLs within its config.xml. This means if it has an entry like this ...
<access origin="*" />
... it will attempt to open all links inside its own webview. However if you constrain your whitelist to only specific URLs, then any link to a URL not in that list will automatically open in an external browser, not within your local webview. For example if you constrain it to only this ...
<access origin="http://127.0.0.1*" />
... then the twitter link mentioned in the original question should open in a new external browser.