Madara paralyses Hashirama with chakra rods. He then proceeds to absorb his Senjutsu. He mocks Hashirama and says that his chakra is meagre and easy to control.
However, we all know that Hashirama is no idiot when it comes to war. Did Hashirama restrain his Senjutsu chakra somehow and fool Madara, or was Madara saying the truth?
Like you've said, Hashirama is no idiot when it comes to war. The case is exactly same for Madara. Madara was a genius at manipulation, tactics, and logical thinking.
Senjutsu chakra is highly potent if a user doesn't know how to handle. But this is Madara we're talking about and not an inexperienced Genin.
Senjutsu practitioners learn to draw the energy of nature inside of them, blending it with their own chakra.
Since Senjutsu chakra is therefore mixed with the user's own chakra. So there is no way to separate it and/or hide/seal it away within a person's body.
And moreover, Madara had Hashirama's cells, welcoming the Senjutsu chakra into his body. Which explains how it readily accepted his body. Moreover, Madara was able to decode it within seconds, and figure out the working of it, hence the comment about belittling Senjutsu chakra.
Even if Hashirama tried to fool Madara, I'm sure he would have realized it within moments. But there was no mentioning about it by either of them.
Hashirama did not fool Madara. It just didn't probably blend in well with Madara's chakra. Madara at full power might have been able to fully utilize it.
