There is a difference between בו and בתוכו (inside it): Whenever it says ששם דבר בדבר (there is something in something) the thing does not have to be inside. But when it says שיושם בתוך דבר (placed inside something) – if it is referring to the vessel, it will be surrounded by three sides, while if it is referring to things that are grouped together, the word תוך will mean at the midpoint.

And this is what differentiates תוך and קֶרב - the word קרב is not strict in that it will be at the midpoint or surrounded all around. And the word קֶרב comes from the perspective of water inside the stomach or food inside one’s cavities. But תוך is not correct with this, since there is no barrier and no midpoint in it.