Neither side takes him seriously, of course. In this historic moment of Islamic uproar across the entire region, Hamas is just following the larger script: act up and act out.
They would like to catch the momentum of rampaging ISIS next door, but Hamas is not a mobile force of mostly young male psychopaths. They're stuck in Gaza embedded among their women and children doing what they can to eliminate Israel and replace it with an Islamic state. For the moment that means lobbing rockets from launch sites planted among the homes and institutions of daily life in the densely urbanized Gaza strip.
So far in the current offensive, they've launched over 700 missiles at Israel. Some of the rockets, purchased through the world's arms bazaar from China, are powerful enough to reach Tel Aviv and Jerusalem. But they are poorly-targeted and Israel's "iron dome" anti-missile system can intercept some of them before they land on anything.
Unlike previous offensives, when Hamas rocketeers could set up mobile Qassam missiles in alleyways or rooftops, and beat it a few minutes after firing, the new larger rockets require sturdier installations. Thus they can be systematically discovered and targeted. Hamas is quite willing to sacrifice the women and children who eat and sleep around their rocket installations for the propaganda value of pretending that they were not deliberately put in harm's way.