The Sonos 5 has the line in connection only, and limited to stereo.  It’s not actually designed for use with TVs.  If you want atmos audio, you’ll need to get the Sonos Arc or Beam.

Would the sound in stereo from Sonos 5 be better than the atmos audio output from the tv speakers

Only you can judge that.  However, I personally would never use a Sonos Line-In as a TV source.  It’s got a 75ms buffer, so it’s always going to be lagging behind the video, causing a lip sync problem that would drive me crazy.

I don’t think that TV actually has atmos speakers built in.  Specs say that it has 2 10W speakers.  What I believe Sony means is that can stream atmos audio sources to your atmos capable audio system.

That TV would love an Arc a Sub and a pair of Era-300s.

If the room is smaller an Atmos capable Beam, mini-Sub and Era 100s would be good too.