I'm about to get General Organic's GO kit, and I have plants in all different stages of life, sprouts, veg, and flower, and I'm not in the least worried about using it on all of them when it gets here, despite that I've used different kinds of nutes (Tiger Bloom and Jacks Classic Blossom Boost for my flowering, ammonium nitrate ferts for veg, cal mag for both). I'd go with the half strength rule like mentioned above but you shouldn't have any problem, I'm sure your plant will thank you for getting off of Miracle Gro! Personally I avoid Miracle Gro products like a plague when it comes to cannabis. But thats just me.
My bet is they just say that because they want you to use more of their products haha, not because it will cause any significant harm to your plants.