The SoundBar is not detected by Spotify. Both the 'requestee' device (such as PC) and the SoundBar (player) are on the same network. Connectivity is also good, as LG Mobile App detected the SoundBar just fine, I can also ping its IP address.
Seeing all those services being up and running on their respective ports, I was puzzled why Spotify Connect cannot detect the SoundBar. This is when I decided to run some more tests with Wireshark logging enabled.
To get some 'success' example logging, I also started Spotify on my LG TV (which gets detected from PC).
Now, the PC-Spotify appears to be polling the local network via a broadcast and then doing an HTTP requests to detected player hosts, in this case my LG TV and LG SoundBar:
192.168.0.100 (LG-TV): HTTP 242 GET /apps/com.spotify.Spotify.TVv2 HTTP/1.1 -> HTTP 200 -> XML with device info was received
192.168.1.199 (LG-SBar): HTTP 242 GET /apps/com.spotify.Spotify.TVv2 HTTP/1.1 -> HTTP 404 -> File Not Found
So there it is, PC/Android Spotify apps, while doing discovery, appear not to follow some rules of the Spotify Connect protocol, as described in the guidelines written by Spotify Developers. Instead, a hardcoded URL is being accessed, and 404 means the SoundBar wont be detected.
Some devices do not use 'com.spotify.Spotify.TVv2' service name for Spotify. Even better, the '/apps/' folder can also change (to 'dial' or 'service' or 'api', etc.), so this should be detected as well!