What the parent posters are saying is that there's no way to trust that your (encrypted) data won't end up landing on the hard drive of someone who chooses to capture everything that passes through said hard drive.
Whether the node itself keeps the data is kind of immaterial, if there's no technical limitation preventing the owner of the node from keeping your data.
But also, as an open-membership decentralized network, there's also no real way to incentivize node implementations to obey deletion requests.
It's helpful to think about peer-to-peer networks as if every node was programmed from scratch by the person or organization running it, to do exactly and only what that person/organization wants it to do. (Not true, obviously, but the incentives behind there being a free market of p2p node software work out similarly to if it was.)
In such an environment, why would a given node owner want to add a "read a deletion-request log, and actually-delete everything mentioned in it" feature to their node in particular? Especially if they weren't a party with their own private data stored that they were interested in ever deleting, but instead were only on the network to make money hosting other people's stuff; or because they wanted to insert public data sets onto the network?