Yes they are just working those leg muscles and it is a bit of a reflex. Babies are born with the ability to crawl but the head etc. weighs them down, they lose the reflex and then have to rebuild the skill. My girl was an early mover and did this a lot and kept doing it until she finally was moving. My boy is also doing this but can tell it’s more of a reflex.
My LG has been doing this for ages we were so shocked as it literally looks like she is trying to crawl, we did ask our HV and she said it's normal. Could be an indicator for early an mover but equally may not be!
My LO does this too, I’m pretty sure is just normal development as they learn to use their legs