Will an external USB 3.0 hard drive connected to an Elgato Thunderbolt 2 Dock run at USB 3.0 speeds even though the Macbook Pro is USB 2.0 only?
The device you're mentioning, the Elgato Thunderbolt 2 Dock, provides USB 3.0 ports. As commented by someone else, Thunderbolt is a PCIe extension and thus is not limited by the USB 2.0 bus of your laptop. As such, it is possible for USB 3.0 to be something that is offered by a dock like what you mention.

USB is a feature offered (typically) by the Intel chipset and tends to be driven these days off of one of the PCIe lines from the CPU. Your computer tends to have a controller that translates the PCIe line to a USB interface (in the CPU's supporting Intel chipset) and would support only the specification that it came with. For example, a MacPro3,1 (2008) uses an Intel chipset that supports USB 2.0 devices.

An easy upgrade for those machines is to install a PCI Express card to gain support for USB 3.0 similar to what you'd do with the Thunderbolt dock, i.e. a USB 3.0 controller that sits on the PCIe bus. Of course, any devices on the PCIe bus needs to be recognized by the operating system and thus requires drivers. However, drivers for some USB controllers (e.g. the FL1100) are supported by macOS / OSX 10.8+ and will just work.

Of course, the USB connector does slightly differ between 3.0 and previous specs; the 3.0 connector has a few extra pins that are key to the higher speeds.