TL;DR: Thunderbolt 3 uses the USB-C connector and includes USB 3.2 signaling (including a DisplayPort signal), but a USB-C connector does not necessarily include support for Thunderbolt 3.
Few things to understand here.
- USB and Thunderbolt are technology standards, both of which establish their own connectors, communication and connection protocols, power delivery standards, and other specifications. Both USB and Thunderbolt support carrying multiple different types of data/power.
- USB and Thunderbolt both have multiple revisions—1.0, 1.1, 2.0, 3.2 Gen 1x1, 3.2 Gen 1x2, 3.2 Gen 2x1, 3.2 Gen 2x2, 4.0 (upcoming), and 1, 2, 3 respectively. Don't worry, everyone hates the USB names as much as you do, and yes, they did change it on you.
- USB-C is a specification for a physical connector type (like USB-A, USB-B, mini USB, and micro USB), not a protocol in itself. While the physical connector does allow/limit what can be carried, it does not in itself specify what if carried.
- USB-C can carry (or not carry) USB data (ranging in speeds from 2.0's 380 Mb/s to 4.0's 40 Gb/s (upcoming)), power, or any one of the display-oriented "Alternate Modes" (DisplayPort, MHL, HDMI, and confusingly, Thunderbolt Alternate Modes respectively.)
- Thunderbolt 3 uses the USB-C port. USB 3.2 supports it, but does not require it, except for USB 3.2 Gen 2x2, which requires it.
- The upcoming USB 4.0 is effectively identical to Thunderbolt 3, but it's not out yet anyways, so who cares.
- The Thunderbolt display, no matter what, requires a Thunderbolt input, not just a display input, and will not function with just a display input. Basically, the Thunderbolt display is a Thunderbolt dock with a display attached to it, with no way to attach just the display part.
- Although Windows tends not to play nice with Thunderbolt displays (and 2011 and newer iMacs in Target Display Mode), they will work with some finagling.
- The USB-C port on the RTX 2080 implements VirtualLink, a proposed USB-C alternate mode that isn't quite the same as the other standardized Alternate Modes, but from a practical standpoint, is often largely compatible.
Whew. Got all that? Let's run through some scenarios.
- Works (usually): Connecting a Thunderbolt Display to a Thunderbolt 2 enabled Windows computer (including Macintosh in Bootcamp.)
- Works, but only under macOS because the adapter still doesn't work under Windows for reasons that I really don't understand and Apple couldn't seem to explain to me over the phone either, and even then only after updating macOS to... some version of Mojave not sure which: Connecting a Thunderbolt Display to a Thunderbolt 3 enabled computer using Apple's Thunderbolt 3 to Thunderbolt 2 (not USB-C to Thunderbolt 2!) adapter.
- Probably, but might not actually work: Connecting an Apple Cinema Display to your RTX 2080 via it's USB-C port using an adapter.
N.b.: Different USB-C ports, as well as cables and devices, can support different parts or levels of the USB specification. Make sure everything in the chain is up to spec!
Still confused? Understandable. Thunderbolt 3, as frankly a godlike specification as it is, is massively confusing. The split between various individual companies, consortiums, regulatory/specification bodies, implementors, vendors, and most of all, the confusing marketing and specification of USB-C makes this an absolute mess to understand. I don't blame you for thinking this would work, but sadly, it won't.
So, my advice? You could go ahead and throw a Thunderbolt card in your motherboard, assuming you have one that supports it (or are willing to switch to one that does.) However, the easier solution is probably just to get a different display.