All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger.
The hadeeth in reference is authentic. The words of the Prophet “..... I would have commanded them to use the Siwaak” constitute a command that denotes obligation rather than recommendation. Ibn Qudaamah wrote, “It means: I would have made it an obligation on them; because difficulty is associated with an obligation, not with a recommendation.” [Al-Mughni]
The Prophet, sallallaahu ʻalayhi wa sallam, feared to overburden his followers by commanding them to use the Siwaak before each prayer as an obligation. This would have been difficult for them, and thus many of them would not observe it and thus incur sin for neglecting an obligation. So he made using it recommended rather than obligatory.
Allah knows best.