I have never come across anyone who claims that there were no semitic arabs in Syria pre-Islam. Unless they are synonymously using "Arab" and "Muslim" (Mistakenly, as the majority of muslims are not arabs) - and that Islam spread into Roman controlled Syria with the Muslim Conquest. Although the majority pre-Islam may have been Indo-Iranians due to military acquisitions made by the Achaemenid Empire, but there certainly was semitic arabs within the modern day borders of Syria during pre-Islamic times.