It is as simple as that. I just did a run and didn't get any rewards. This is the normal Onslaught mode.
Update:
I am getting rewards now. I ran again two times and got rewards. It must have a temporary bug for that first run today
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I didn't get anything when I completed Onslaught, but I only did one match. I'll try again later and see if I'm still getting a lootless chest.