Translation:If they were harder, we would not be able to eat them.
Just to clarify, this is about the second 'ze' in the sentence.
I think there's more to this. And although I'm not a linguist, I'll have a go at explaining this.
'Ze' can be used for everything including people. (Also in the sentence above.) 'Hen' only refers to people.
Distinguishing between people (hen) and objects (ze), may formally be 'more correct', but in practice you hear both and I wouldn't say it's wrong.
Example: Don't let them wait! -> Laat ze niet wachten!
This is perfectly fine. You could also use 'hen' to sound a little more formal.
The only case I can think of when you pretty much have to make the distinction, is when 'them' is used as an indirect object (meewerkend voorwerp).
Examples: 1. I give the ball to them. -> Ik geef de bal aan hen. 2. I will buy a gift for them -> Ik zal een cadeau voor hen kopen.
In these sentences you can use 'ze' to sound a bit more casual and a little less formal or stiff, but you can't stress the 'ze' and preferably you should speak quickly and confidently. If you don't, it sounds a little off :)
I'd like to hear what other native speakers think!