The downloadable shortcut described here also worked for me.

I assume you’re referring to Safari on iOS. I was able to select all on that Project Gutenberg page with a little-known scrolling trick:

1. Scroll to the bottom of the page. Yes, this part is a bit annoying but I was able to do that in 8 seconds with 25 full-screen flicks.
2. Long-press near the bottom of the page to start text selection.
3. Grab the bottom lollipop and drag it to the end of the page to select the last character.
4. Grab the top lollipop and drag it around a little to select more text. Don’t release it, and hold it still.
5. With a different finger, tap the status bar at the top of the screen. This is a shortcut for scrolling to the top of the page. Give it a couple seconds to finish scrolling. If you move the lollipop at all while it’s scrolling it will interrupt the scrolling, so keep that finger still until it’s done.
6. Now that you’re near the top of the page, drag the lollipop to the very top of the page and release it. The copy option should appear.

There’s no way to prove that something is secure. (It reduces to the halting problem.)