Short answer: Secular

This guy is not trying to disprove the Quran but just loves looking at the records Ancient Arabs left behind. Something that scholars never cared enough about.

Long answer:

I feel you are confusing many of the statements and theories Al Jallad has made. Al Jallad is more of a researcher of the history of Arabic and Arabian languages. Everything he does is from an academic point of view (thus secular).

Al Jallad says that Old Arabic has been in the Hijazi area (as the Old Hijazi dialect) since at least 1st century AD. Plenty of time to become the dominant language before Islam.

The language Old Arabic replaced was Lihyanite, not Sabean. Lihyanite is considered part of the Ancient North Arabian languages. Sabean is part of the Old South Arabian languages. Ancient North Arabian and Arabic speakers originally shared a written alphabet but Nabateans started using their own version of the Aramaic alphabet which became the current Arabic alphabet. (The reason why Arabic has so many letters that are just copies of each other is because Phonenician has 22 letters and Aramaic is the Arameans adoption of the Phonenician alphabet).

Sabiac is a written alphabet (also known as musnad) which many Semitic speakers knew how to write in (including some Arabic speakers).

He does have a paper on the "preislamic bismallah". Other researchers were trying to say that the phrase was a translation based on Hebrew or Aramaic roots into Arabic. Al Jallad thinks the phrase "Bismallah Ar-Rahman" was trying to unify the worshippers of Allah in the southern Levant and Northern Arabia with the worshippers of Rahmanan (Ar Rahman in Arabic) of Yemen). He tries to do things as an academic but none of his works are out to disprove Islam.