Would context matter?
I dont think that's strictly true. I think purpose in could be followed by a gerund.
For example, someone could take a sentence with a fairly simple structure like "she swims purposefully" and change it to the convoluted "there exists purpose in her swimming."
But I know what you mean.
You cannot say, "What is the purpose in the handsaw?". You can say, "What is the purpose of the handsaw?".