Wa aleykum as Salam wa rahmatullah,
I am not a teacher, however the way i understood it is as follows:
The reason, why "Allah is one" is not a morally necessary statement. I assume, that it is due to the statement not being a command, not being directed to someone, who is morally responsible (Mukallaf) and it does not make an action required, interdicted or allowed. Since the definition of a legal/moral ruling is: "The legal ruling (hukm shar`i) is the command of Allah to those morally responsible to make an action required, interdicted, or allowed.
As for the statement "to believe Allah is one": There exists various Ayat of Quran, wherein Allah commands us to believe in his Oneness. This command is directed at the morally responsible from his servants. And believe is an action, that is required. Therefore the statement "to believe Allah is one" fits the definition of a legal/moral ruling and is for that reason a morally (shar'i) necessary (Wajib) one.
And Allah knows best.
I hope, I could help you and I ask Allah, that he increases us in knowledge and makes us from his humble slaves.