Ren Xiaosu was only able to go head-to-head against the terrifying creeper vine by relying on the armor. Without it, Ren Xiaosu would have found it very difficult to handle the inescapable spikes on the feelers.
But the armor always had a flaw—limited power.
It was way too slow to solely depend on the bioenergy of the human body to recharge its energy. Moreover, the nanomachines were too small in size, so the amount of energy they could store was also extremely limited.
Therefore, Ren Xiaosu's armor was unable to sustain much longer after a lengthy and intense battle. Thus, he told Zhou Yingxue that she only had one chance.
If they failed this time, they could only continue fleeing for their lives and try their best to get out of this place quickly.
Although he still had the steam locomotive as his trump card, it was not an omnipotent solution. Ren Xiaosu was worried the massive steam locomotive would get entangled in the vines and end up stuck.
Everything is One: From the Buddhist philosophy of (inter)dependent origination | further reading: https://en.wikipedia.org/wiki/Prat%C4%ABtyasamutp%C4%81da#Meaning | https://en.wikipedia.org/wiki/Impermanence | https://en.wikipedia.org/wiki/Substance_theory