Bear with me but I believe that the holes use water pressure differences to circulate water. The water pressure toward the outside of the volute is much higher than in the impeller chamber or toward the middle of the volute, so if you put a hole toward the outside and a return near the middle, water would circulate into the outer hole and out the inner hole.
Whether their particular fix did the trick, I cant say yet, but mine is still working well at this time. Maybe in another month or so I will disassemble it and have a look.
I see the comments of wanting to go with Abyzz for the heart of the system, but unless your return pump is impossible to remove/service, I just cant see paying 3x as much for a similar return pump (actually the ecotech has more features). Heck you could buy two of these and still come out ahead. But I understand if you need even more flow or want to reduce power consumption, then Abyzz could be the way to go.
Interesting to read that some folks running these internally have not had such issues.
Either way, no matter what pump we all choose, it is nice to have a company like ecotech actually fixing their issues instead of just living with a high return rate.