Bullet is great for honing one's tactical instincts.
But there are long-term positional/strategic considerations (crucial for playing well in long games) that can't be practiced to any decent degree in bullet, nor in blitz.
So, yes—I partially agree. Bullet is great for improving some things. But it doesn't cover everything.
This may sound like a deliberate mis statement but it is true that in order to improve your overall game it is absolutely essential to play bullet seriously. Magnus Carlsen himself has said that he invariably knows which move he is going to play in a classical game in the first ten seconds. He just spends the rest of his time confirming it. It is also known that he is the strongest bullet player on the planet as are the other top classical players. Bullet utilizes pattern recognition and tactical ability like no other form of the game hence it is essential that we learn to play quickly, to see things quickly to calcualte in a moment and see instantly all variations and therefore bullet is essential to improvement and is a deadly weapon in the hands of a classical player. I have spent the last two days playing bullet and today I beat two consecutive games an 1750 player in rapid. Something that I would normally struggle to do.