The mortar attack on the shore-based component of the temporary humanitarian pier in Gaza being constructed by the United States is likely an act by Hamas, though no group has claimed responsibility. There was some misreporting of what took place as an attack on the US ship off Gaza's coast building the JLOTS (Joint Logistics Over-The-Shore) pier when, in reality, the incident took place on Gaza's coast as UN staff were inspecting the site with IDF troops.
The message behind the attack is simple: Hamas is unwilling to tolerate humanitarian aid avenues that it does not/cannot control. This is especially so given that, for now, it appears certain that the Israeli military will be providing security for the arriving aid, which will disembark using the 'causeway' that'll be anchored to Gaza's shore. It would be a mistake to rely on the IDF long-term for providing security, as this will increase the likelihood of the port facilities being targeted by Hamas and other militants. Instead, an Arab-led contingent should involve the Palestinian Authority to provide on-shore security and act as an intermediary between arriving aid and the local population.
Another reason why Hamas is nervous about the port becoming operational in the next couple of weeks is the group's concern that a Rafah invasion is more likely with an alternative avenue for aid going into the Gaza Strip. There is a high probability that the IDF will take the "Philadelphi Corridor" along Rafah's borders with Egypt, taking the border crossing out of commission, at least for some time. This will close off the primary artery of humanitarian aid into Gaza, a concern that has delayed the Rafah operation until an alternative is activated. Hamas views the operation of the US-built humanitarian pier as yet another event that could make it easier for the Israeli war cabinet to proceed with a Rafah assault.
Notably, the port will be located just north of the IDF-controlled road/highway that bisects Gaza's northern sector from the rest of the coastal enclave. This poses several questions: Will the pier mainly supply the north, where less aid goes in, and a serious hunger/malnutrition crisis has been unfolding? Or will it be used for the entirety of the Gaza Strip? How will aid coming through the pier be trucked down south to Deir al Balah, Khan Younis, and Rafah, given how the trucking mechanisms/logistics points are concentrated around the Rafah border crossing?