Yes, why wouldn’t it be stereo, when the speakers are set up on the network as a stereo pair? Now, if they were set up as two ‘rooms’ and grouped, rather than a stereo pair in a single Sonos ‘room’, then they’d both be playing the same combined right and left signal. 

Try and play "Heard it through the grapevine” by Marvin Gaye. You'll hear the stereo effect in the first seconds.

By the way, if both speakers would play the same sound it wouldn't be stereo but (dual) mono. There's no "true” vs “untrue” stereo.

I can confirm, when in range of their WiFi connection, that both the Era 100 and Era 300 paired speakers play in stereo over a Bluetooth pairing - I tested with a stereo test file from Amazon Music - Also, it does not matter if the BT sending device (an iPad in my case) is BT-paired to the left, or right, speaker, the correct/separate audio channels are played on the Sonos stereo pair. The left audio channel goes to the left speaker and the right audio channel goes to the right speaker.

Thanks for everyone’s helpful response! Now I don’t need to worry about this issue and go head to buy Sonos speakers as my plan.