Both in slow motion look 100% red cards.
I think both players are genuinely competing for the ball and not trying to endanger the opponent. Dele was just quicker to the ball than Sterling, impossible for Sterling to react to pulling out once he's already decided to commit to the challenge in the first place.
My biggest issue with the Sterling tackle, however, is that it should have been a red card given the president that went before it. There were at least two challenges (Aubameyang against Palace and I can't recall the others) where they were sent off for very similar tackles. Based on the similarity the precedent has been set and Sterling should have got a red card 100%.
But without those that went previously, I don't think they are red cards, the same as I didn't think Son's was. I get that things can become a bit blurred at times, so to complicate it further I thought Pogba should have been sent off for his challenge on Dele(?) when he too I believe was trying to make a genuine challenge for the ball and was late and brought down his studs on the thigh. So why do I think this was a red? To me there is a bigger distance between the players, he is significantly latter than the others mentioned, so he has time to pull out, his foot is high and whilst the high foot stuff is subjectively applied he missed the tackle and did endanger the opponent.
Fine line sand all that.
FWIW: The officials did confirm what I thought their interpretation of the original decision might have been: