[From the article]
“And for those members of our military in the chain of command, here in Washington or overseas — look very carefully at every order given to you by this president,” Moulton said. “It is our duty to make sure everything done in the name of the United States military is lawful; and that burden falls on us. ... We have an opportunity to call on our military officers around the world to bring new scrutiny to their sworn oath to obey lawful orders, and only lawful orders.”
While you are correct that Moulton is not explicitly saying that he thinks the order to kill Souleimani was illegal, he is certainly implying, rather strongly, that EVERY ORDER given by Commander-in-Chief Trump is suspect, and thus must be “look[ed at] very carefully.”