فَالضَّيْفُ وَالْجَارُ الْجَنِيبُ كَأَنَّمَا هَبَطَا تَبَالَةَ مُخْصِبًا أَهْضَامُهَا
The guest and the unrelated neighbour are as if they had descended into fertile Tabālah, whose valleys teem with abundance.
Al-jār al-janīb is a neighbour or guest outside the close kin group. Tabālah was associated with a fertile region, and ahḍām refers to the lower parts of valleys.
The simile turns the generosity received by a guest into the experience of entering a rich and productive landscape. Kaʾannamā explicitly signals the comparison.