This hip flexor stuff is an odd business. So when you stand up straight, your thigh/torso angle is 180°. When I'm on the bike, my thigh/torso angle seldom exceeds 90°. What on earth would be the point of stretching the hip flexors beyond 180°? It makes little sense to stretch joints beyond the range of motion to be used in the targeted activity. I know that bro science and all the spin classes, etc. advocate that stretching, but I see no sense in it and I do don't do it. OTOH, strengthening the hip flexors is sadly neglected in cycling training. I work my hip flexors good in the gym and that definitely helps. On the 3rd hand, the only nasty injury I ever had in my athletic life was overworking or overstressing a hip flexor tendon when I raced Nordic in college, so both stretching and strengthening are indicated for that sport. Took me a couple years to heal that up and I still have to be careful of it. For cycling, sets of 30 bent-legged leg raises on the Roman Chair, or do fewer reps holding dumbbells between your feet, better yet.