Most Mediterranean trade around the time of the first crusade went through alexandria and constantinople, making most levantine cities minor tradecenters at best. Note that this was mostly the result of established traderoutes and not because of some fatimid or byzantine effort.

His claim mostly stems from the fact that the heartlands of the two biggest Muslim empires were Egypt and Mesopotamia (and later anatolia). The only big tradecenter in the levant was a Aleppo which enjoyed autonomy but was further inland Syria.

I believe his remark was mostly toward the fatimids which mainly looked at the seljuks as their rival, not expecting a western army to suddenly challenge their rule of the levant. Their administration and trade was done through Alexandria and egypt and an incursion into the syria and Palestine wasn't seen as an existential threat.

Asbridge actually goes on to explain why a lot of the levant developed into bigger trade enters after the third crusade and the biggest factor is mostly Italian and other european merchants moving to outremere.

I would say the reason the levant was a frontier region was because middle eastern rule was already unstable in the 11th century and its power bases were hard to defend.

My sources are Asbridge's book itself and the Edinburgh histories of the fatimid and seljuk empires.