He is making a comparison between Greece shape, and a specific tree leaf (in this case the “Platane” i.e.: “Plane tree”). The construction of his sentence is quite intricate. To help you read it, here’s a split of the different statements it contains:

• Ce port est situe dans l’une de ces trois profondes indentations des côtes de la Grèce méridionale.
• Ce port est l’ancien Oetylos d’Homere.
• Ces indentations découpent (dessinent), sur la mer Ionienne et sur la mer Égée, une feuille de platane.
• La Grèce méridionale est très justement comparée à une feuille de platane.

It translates to:

This port, the ancient Oitylo of Homer, is located in one of the three deep indentations that trace, on the Ionian and Aegean seas, the plane tree leaf to which southern Greece has justly been compared.

It feels even heavier in English than in French.

With a restitution of traditional language forms (I’m not at all aware of this apparently new way of writing Homer in English, I’ve never found any other form than "Homer") and a little inspiration from the preceding I come up with the translation below.

Ce port, l’ancien Œtylos d’Homère, est situé dans l’une de ces trois profondes indentations qui découpent, sur la mer Ionienne et sur la mer Égée, cette feuille de platane, à laquelle on a très justement comparé la Grèce méridionale.

This port, Homer’s ancient Oitylo is situated on one of the three deep indentations that the Ionian and Aegean seas carve out of the land in the shape of a plane tree leaf, a shape to which is rightly compared Southern Greece.