«كَأَنَّهُ عَلَمٌ فِي رَأْسِهِ نَارٌ»
As if he were a prominent landmark with a fire at its summit.
The previous assertion already established Ṣakhr's guidance. Comparing him to a conspicuous landmark makes that quality visible; adding “with a fire at its summit” intensifies the image further, combining height and light.
The closing detail is unnecessary to the basic commendation but supplies heightened praise. Its function is therefore a further nuance at the close rather than completion of an otherwise missing proposition.