Zaraki gave that explanation in Chapter 105, page 7. However, just a few chapters later (chapter 109, page 19) Zaraki chides Ichigo for relaxing and thus allowing his Reiatsu to soften. Enabling Zaraki to easily stab Ichigo.
It would appear that people who like to rage against Kubo do so based on selective pieces of information. All they would have to do is keep reading from the point and they would see that it IS more then possible for a lieutenant to stab a fully hollowfied former captain when said former captain was focused on charging a blast to kill Komamura and thus softened his Reiatsu because his opponent was defeated and lying in front of him.
In short, because Tosen believed he had already defeated Hisagi and Komamura was lying in front of him. He let his guard down. Thus he was stabbed.