Spoilers for the roku novel ahead!
Apparently in this novel they retcon Iroh inventing lightning redirection to Sozin being the inventor. Now I know the writers have retconned their own canons before, but in this instance I genuinely don't understand why they decided to do this because it makes no sense.
For Iroh to observe, analyze and implement bending techniques from other nations into his own firebending makes sense. This man has spent a lot of time in the other nations, both fighting for the firenation as well as traveling on his own or with Zuko after the latter's banishment. Iroh is also not someone who seems to believe firebending is superior to all other bending, or at least he doesn't give the impression that he ever really subscribed to that ideology. It is canon that Iroh visited the dragons Ran and Shaw before Zuko was even born, as we learn from Zuko: "the last dragon was killed long before I was born, by my uncle" which is the lie Iroh spun to protect them. If those dragons deemed him worthy before he even besieged Ba Sing Se for 600 days, he must have never agreed with Fire Nation supremacy at heart, even if he did participate in it. (Yes i also agree this stuff is a bit bullshitty, but i can at least understand a man like iroh implementing moves he learned from waterbenders in his own firebending)
Now Sozin on the other hand, I would never in a million years believe a man like him chose to observe waterbenders, a people he actively believes is lesser, and then learn from them to develop his own technique. Sozin seems like a man who would prefer to shit in his own hands and clap rather than try and learn anything from any other nation, let alone waterbenders. This man started a war because he believed his own people were the superior. You can't convince me he would choose to learn from people he thinks are nothing more than lowly peasants not worthy of his time. Even if it is to improve his own power, even if it is to invent his own technique.
They also made it so Roku is also capable of bending white hot fire like Rangi, which I also don't know how to feel about. I haven't actually read the novel yet and I'm still planning on doing that, but man knowing these things made me a bit less excited about it.