edit: Found it, uws.js in resources/bootstrap/discord_rpc.zip
edit2: This is arguably worse, but the argument may be "it's a compiled binary":
-bash-4.1$ strings discord_rpc.node | grep uws.*cpp$
../../discord_common/native/third_party/uws/nodejs/addon.cpp
../../discord_common/native/third_party/uws/nodejs/addon.cpp
../../discord_common/native/third_party/uws/src/EventSystem.cpp
../../discord_common/native/third_party/uws/src/Extensions.cpp
../../discord_common/native/third_party/uws/src/HTTPSocket.cpp
../../discord_common/native/third_party/uws/src/Network.cpp
../../discord_common/native/third_party/uws/src/Server.cpp
../../discord_common/native/third_party/uws/src/UTF8.cpp
../../discord_common/native/third_party/uws/src/WebSocket.cpp
../../discord_common/native/third_party/uws/nodejs/addon.cpp
../../discord_common/native/third_party/uws/src/HTTPSocket.cpp
../../discord_common/native/third_party/uws/src/WebSocket.cpp
../../discord_common/native/third_party/uws/src/EventSystem.cpp
../../discord_common/native/third_party/uws/src/Extensions.cpp
../../discord_common/native/third_party/uws/src/Network.cpp
../../discord_common/native/third_party/uws/src/Server.cpp
../../discord_common/native/third_party/uws/src/UTF8.cpp
-bash-4.1$discord_rpc.node is an ELF file. It's hard to see how that would not be considered a binary distribution (if it was that file alone, and not being distributed with source files)
If they are not distributing source code ("3. This notice may not be removed or altered from any source distribution."), it frankly does not matter if they disclose whether they use your library or not. And if they were distributing source code, the notice would just have to stay visible in the distributed source file. An average user wouldn't be able to find that anyway.
A compiled binary != distributing source code.
As a library user under the license you used, there is no legal obligation to include the actual license in (the documentation of) your app. If you wanted that, you should have included that in the license header of your source code. Sorry to break it to you, but they're probably not doing anything wrong here.
https://dl.discordapp.net/apps/linux/0.0.1/discord-0.0.1.tar...
expand that out, then navigate to: resources/bootstrap/
expand out: discord_rpc.zip
note the presence of uws.js which is the source code for this library.
Thus this is a distribution of this library as source, not binary.
Just to be clear, I'm not the original author, and thus have no ability to enforce copyrights for this library. Just someone who doesn't like the easy stuff of license/attribution being thrown away.