Splay leg is usually present at our shortly after hatch. Slip tendon is common in the first week especialy in hatch shipped and feed store bought chicks.
Honestly mine didn't look like slipped tendon either, no notable swelling our falt joint that i could see. But it was a full year ago now and it was a feed store chick that I had a broody hen raising. It took 2 or 3 trying feeling and trying to adjust the tendon before I was sure I did my best. I couldn't see it like they describe I had to feel it from higher up get leg down to the hock, once i found it it was easy to manipulate. You need to gently stretch the leg back similar to a grown hen stretching her own leg, with the leg extended feel for and adjust the tendon. Some treatments suggest suspending the chick in a homemade chick swing. I choose to keep mine on the ground so we wraped a figure 8 with tiny cuttings of vet wrap losely around the joint top and bottom towards the front x over the hock joint. Then stretch at least once a day though 2-3 times are recommended. It took at lease a week for her to heal up but once she did I could no longer tell her from the other chicks. I'll look and see if i can find my post from last year but i don't think i had any pictures.
Truth is your options will come down to try to fix what you think it is, so nothing and probably have a disabled hen (they do learn to function very well), or consider ending is pain. As I'm sure very few people want to go the last route. It's best to try everything you can and hope. With slipped tendon and splay leg the sooner you start treatment the better chance they have. One difference if I remember right is splay leg they will usually hang one or both legs down when held (like they cant pull them up) but with slipped tendon i think it's usually pulled up and they have a more difficult time setting it down (like a sprained ankle, it hurts more to stretch it other then to keep it up)