There is no proof that Garou got stronger in between fighting Bang and doing the 6-C feat. A common point used to back up this argument is that Garou was
able to grow in 0.013 seconds during his fight with Platinum Sperm (Chapter 156, pages 14-15). This does not make any sense, as whenever Garou grows it is usually
directly stated or made clear. This never happened in Chapter 154 after fighting Bang.
While talking to Garou, Gyoro Gyoro even states that the reason Garou is able to grow so much is
due to him surviving near death experiences and other types of stress (Chapter 92, pages 3-9). This is proven over and over, as shown in his
fight with Bang and Bomb (Chapter 83, page 56. Chapter 84, page 1-39), his
fights with Royal Ripper (Chapter 88, page 58. Chapter 90, pages 33-39), and much more. These conditions were not met in his fight with Bang, so it makes no sense to say that he grew. It's completely baseless.
Another common point used to back up this argument is that Bomb was
shocked upon seeing Garou's actions once he woke up (Chapter 154, page 20). Not only is this not enough proof that Garou grew, but Bomb wasn't even reacting to Garou's strength. He was reacting to
Fuhrer Ugly's sudden death despite him being in the middle of an attack (Chapter 154, pages 17-19).