The spell Detect Thoughts has the following limitation on potential targets:
You can't detect a creature with an Intelligence of 3 or lower or one that doesn't speak any language.
Languages Deep Speech understands but can't speak, Telepathy 60 ft.
This seems cut and dry: it says right there that it knows Deep Speech but cannot speak. But I feel that the situation is not that straightforward.
This situation seems similar to a wizard with their tongue cut out. With no tongue, they cannot speak. They have the mental ability to speak, but they lack the physical ability to do so without some magical or mechanical help.
I feel that an Intellect Devourer is in the same situation. When it uses its Body Thief ability to control a victim, it can speak through their mouth - as confirmed by an official D&D 5e product:
It's confirmed by Meloon Wardragon's stat block in Waterdeep: Dragon Heist (p. 210). His languages are shown as "Common, Deep Speech, Telepathy 60 ft." It does not contain the "but can't speak" portion found in the intellect devourer's stat block, but does contain the Deep Speech and Telepathy from the intellect devourer, which makes clear that the stats shown are for the infested version of Meloon.
The intellect devourer possesses the mental ability to speak, but its natural form lacks the physical means of doing so without the help of a host body.