On the same page, in section Perform initial Heads configuration, the last paragraph reads:
Note that during the process of setting up the default boot option, Heads will ask you whether you want to store your LUKS password in the TPM. This is not currently a supported option so stick to the default answer, no, when prompted. You should then boot into your OS.
I successfully installed PureBoot today on my Librem15v4 with PureOS, and at this stage Heads did not ask me whether to store your LUKS password in the TPM. Is this outdated, or is it a peculiarity of my installation process or setup?
I built both Coreboot and Coreboot+Heads images from source using the instructions on the same page, and I already started a couple of months ago to use my Librem key at boot for main internal storage LUKS key unlocking.