The fighters are part of the Brigades controlled by Fatah, the party which also controls the caretaker government in the West Bank. Palestinian Prime Minister Mahmoud Abbas is also a member of the party.If Al-Aqsa has the freedom to have a public graduation ceremony in Gaza, that means that Hamas is allowing their supposed "sworn enemies" to train militarily. The fact that "other factions" attended the ceremony shows that it is probably not only allowed, but encouraged.
The general commander of the Al-Aqsa Brigades and leaders of the Consultation Council (senior leadership of the party who discuss Fatah policy) attended the graduation along with members of other Palestinian factions.
Trainees practiced military shows and skills such as storming buildings and kidnapping soldiers, and demonstrated the launch of rocket-propelled grenades for the ceremony audience.
Al-Aqsa Brigades asserted that the graduation of a class of fighters sends a clear message to Israel that resistance is still a valid choice for freeing Palestinian lands and the Al-Aqsa mosque.
Before the Hamas takeover of Gaza, joint terror attacks between Hamas and Fatah were not unusual.
And while Hamas has shown no compunction about cracking down on groups that show too much independence, Fatah is still allowed to operate freely in some cases.
It seems that while the political wings of Hamas and Fatah are still at odds - Hamas arrests Fatah teachers and civil servants at will - their military wings have no problems with each other, and probably cooperate (along with Islamic Jihad.) This idea is reinforced by the fact that Fatah in Gaza still gets weapons, and all weapons smuggling in Gaza goes through Hamas.
Moreover, Abbas - who is not only a "member" of Fatah but its leader - neither instructs Al Aqsa to attack Hamas nor does he tell them to put down their weapons; rather he is quite happy letting them operate as a terror group together with Hamas and PIJ aimed at fighting Israel while keeping them far enough for plausible deniability so he can get Israel to give him more unilateral concessions.
Just like Arafat.