I'm not 100% sure here, but I think checking this resolved a warning prior to publishing that I was going to lose support for some devices. I will freely admit that it was a flurry of activity trying to figure this out because it was unclear what I had to do at the start. Haha when I first got the notice from Google, I was like, "But everything is already being built as Any CPU!?". I didn't realize there was other options.
My test phone is 64-bit. I think more phones than we realize are... it's not straightforward to find out, apparently. I did some googling around and the first place I suggested said to look at the Kernel version, which I did, and it didn't say 64-bit. However, other articles said to use an app called Antutu, which did confirm that my phone is 64-bit. Here's a link to it.
That app says my phone is 64-bit, and my app ran fine on it. Additionally, I had some other friends test it with their 64-bit phones and everything worked. I'm not sure if there's anything else at play here.
I uploaded the single apk file that contained each abi. I don't know what side loading them means though. Can you elaborate? I'm happy to try a test if it will help you.