Glofish, I don't think it is too hard on ATC. When the EK Crew get a transmission about the traffic they can't understand they say it's unreadable and asked for it again.
When the EK Crew read back the wrong level ATC says nothing. So if he didn't hear the read back he should have asked for a read back of a clearance. If he did hear it he should have corrected it. If he was unsure of what he heard he should have said so and asked them to say again. By not saying anything he confirmed to the crew their read back was correct.
All the time air Seychelles heard all of that and said nothing.
The traffic information call by ATC was at first unreadable by the crew. Remember edge of VHF range. They tell the controller this and ask him to say again. The controller does so, but at the same speed as the first Non readable transmission. You have been there I am sure. You listen in hard, make out what you can, look across at the other pilot who says “”lower after crossing” . Sounds reasonable and makes sense. You reply Roger. After all you read back your clearance and no-one corrected it right?