An indefinite subject can be used to express elevated standing or extraordinary greatness. The speaker invites the hearer to regard the entity as more significant than the bare common noun would suggest.
لَهُ حَاجِبٌ فِي كُلِّ أَمْرٍ يَشِينُهُ
He has a barrier or protection against whatever might disgrace him.
This excerpt belongs to the source's poetry discussion. «حَاجِبٌ» is not simply an unidentified barrier: its rhetorical point can be the exceptional force of that protection. Magnification primarily concerns quality or rank, not the number of objects.