I looked up the program briefly. It seems like standard correct lower body workout. You probably don’t need a bungee cord to do something similar but bungees are a standard thing.
My first question would be, what’s your current off the horse fitness routine? If it’s nothing, then any fitness routine that works on lower body and torso fitness, balance, flexibility and stability could help your riding.
As far as targeting certain exercises for certain position problems, it would be so useful to get individual assessment from a sports physiotherapist even a non horse one, if you could explain the moves and issues. It is also important to see if you have actual skeletal issues like scoliosis or one leg shorter etc.
It’s also true that any fitness program is only as good as the effort you put into it and many of us can’t be trusted to follow through with any at home physical exercise
for the long term.