The way I look at it (again, total amateur here), the article ὁ helps indicate that "the Garden" is a proper noun, the name of some unique place, as opposed to a "garden", which could be one of many. I imagine that Athenian Epicureans could have casually disregarded the article, but it's sort of like ... "The Beatles". You'll always put "The" in front of the brand unless you are referring to each "Beatle" individually, or the group of those four people as "Beatles", but the band, proper, is "The Beatles", and it indicates more than just a group of four people; it indicates their entire brand.
Thanks @Eikadistes!
On a slightly related note, it seems that many authors were in the habit of using the plural.
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Heraclitus (fl. f. 50 CE), Allegories of Homer, 4
ὁ δὲ Φαίαξ φιλόσοφος Ἐπίκουρος "ὁτῆς ἡδονῆς ἐν τοῖς ἰδίοις κήποις γεωργὸς"
Phaiāx calls the philosopher Epicurus "the cultivator of pleasure in his own gardens"
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Seneca (fl. 35 CE), Letters to Lucilius, 4.10
Accipe quod mihi hodiērnō diē placuit – et hoc quoque ex aliēnīs hortulīs sumptum est:
Receive what has pleased me on this very day – and this too has been taken from little foreign gardens
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Seneca, Letters to Lucilius, 21.10
"ecquid bene acceptus es? nōn irritant" inquit "hī Hortulī famem, sed exstinguunt
"were you well received?" he says "these little gardens do not provoke hunger, but they quench it"
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Cicero, On the Laws, 1.13.39
in hortulis suis iubeamus dicere...
In their small gardens let us allow them to speak...
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There are more from Athenaeus and Plutarch.
