Storm of Iron was written way before the HH series came out.
There were no Grand Battalions back then, the Iron Warriors had Grand Companies pre and post Heresy.
So there's two options; 1) an in universe explanation where Grand Companies are degraded remnants of the Grand Battalions. 2) Replace every instance of the phrase 'Grand Company with 'Grand Battalion', as the fluff has evolved since then.
There were no Grand Battalions back then, the Iron Warriors had Grand Companies pre and post Heresy.
So there's two options; 1) an in universe explanation where Grand Companies are degraded remnants of the Grand Battalions. 2) Replace every instance of the phrase 'Grand Company with 'Grand Battalion', as the fluff has evolved since then.