Hey @MatthewGuilliatt
For new installs
You can try the link below. This is the link we have behind the "Download on the App Store" button. This will take them directly to the App Store after-which they should be able to open the Explorer app link you have in your original question.
https://itunes.apple.com/us/app/explorer-for-arcgis/id860708788
For users who already have it installed
For those who already have the app installed, you can try a couple different ways to get the OS to open Explorer by default without uninstalling and reinstalling.
1. Tap the blue OPEN button in the Smart Banner seen in the image below. You may need to swipe the page down for the Smart Banner to appear
2. Press and hold on the app link from an email, text message, etc. This will present the option on how you'd like to open the link. If you chose the option "Open in Explorer" it should remember this for the next time. Here's an example of me pressing and holding the link from a note in the Notes app.