I imagine a redwood is bigger than said pump in all particulars. You've been presented with a nicely detailed explanation. Before demanding a "better" one, you might explain something about what was wrong with the old one.
If water getting to the top of a redwood is a mystery, water getting to the 35,000 foot level in the atmosphere is a mystery beyond comprehension.
I agree with you. It is all amazing. As for the water to the top of the Redwood, I will first say that I am not a scientist. My roomies in college were all into science, working on their masters degrees and some into such things as rapid cell division to try and get a redwood to grow faster. It was they who explained to me that it was a complete mystery how a redwood tree can get water to the top.
The explanation you say was offered does contain the word believe. I don't think it was purporting to know the exact answer, an admission that we simply don't know. We know some of what takes place, but we don't know how.
Happy to hear how I am wrong on this.