It's a sideload APK so that you know exactly what you're downloading. Google Play search can often give you malware with an identical seeming name and icon to real apps. Since this app requires root, it's very important to know exactly what it does - that is why it is OSS and I highly recommend anybody who knows Java to double-check that the app is non-malicious.

Relying on Google Play for a root-requiring app is not in any way a guarantee of safety - you can upload a malicious app to Google Play, and it can sometimes be as much as 24 hours before it is taken down.

Correct method is to download the source from github, verify it is correct, and compile and build your own version. Failing ability for you to do that, get someone you trust to verify the code, and failing that, try and make sure that there are no comments about the app being malicious.