Reread the passage. No blunt damage can hurt her. Says nothing about cutting or piercing attacks. Blunt damage would be punching. Deep forest emergence is piercing and tearing. It also specifically mentions when the technique is activated, meaning it's only a factor when she's in the process of dividing.
Deep Forest Emergence is not a cutting attack. It is the creation of a forest that allows hashirama to use mokuton for attack or defence or capture, but we have never seen it be used as an attack for cutting anything. Im willing to accept if you show me scans though.
Also, it doesnt matter if you piere this huge gigantic katsuya. Any piercing damage that hashirama can do to it is minimal at best, when the scale of katsuya full body realized. It can also simply heal itself using sakura.
So there is no damage that Hashirama can do to a Katsuyu that is the size of the Shinju Stomp.
Ill try to put into scale so that you can see how large it is:
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Now this in scale shows how large hashirama is compaired to the Shinju stomp.
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and
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. Katsuya on the other hand would be far larger then the main roots shown in this scan.
So if you can concieve this scale then you can clearly see, hashirama is an atom to the likes of 100% katsuya, and his Deep forest emergence is just grass under her feet.