This is exactly where I sit with it too. I've watched the video and unless I've seen a heavily edited version I can't see where anyone is pushed or shoved into the road?
You can see it enough certainly on most of the videos I've seen, but it helps to full screen the video on a monitor rather than phone screen and scrub through it slowly at the point they pass because there isn't a clear video of her planting her hand on her.
You don't see her hand make contact, you do see her hand hesitate and her feet change pace, her arm travel up, her weight shifted forwards, her her arm come back into frame, all at precisely the correct timing for Celia's front wheel to suddenly turn off the pavement (despite already having passed her) and then twist and fall as if force had been exerted on the shoulder closest to Auriol.
Add the context of her swearing, and maintaining her position that Celia shouldn't have been on the pavement, and the admission of contact, and even if you can only sort of see a push, it's enough for me and plenty of others to feel confident that based on the footage, context, and understanding of human motion, that she pushed her enough to knock her off balance, even if lightly.
If she hadn't admitted to contact I might have believed that she just flung her arm out and startled her, but that's by the by, excuse she did.