That particular TV does have a "Center Channel Speaker" input on the back, in the form of spring clips. It's a single channel connection, that drives both the left & right transducers in the TV.
That doesn't help PSS, since the customer seems to want no other speakers added. So missing the L&R audio. Hence the need for preamp outputs from an AV receiver, going into an HDMI audio inserter, which would then go back into the TV for the front L&R audio to come from the TV (using TV power). Then the rear surround in-ceiling speakers powered from the AV receiver.

In Mike's case above, it was being used as intended with external power driving the TV transducers as a center channel. And being it was stealth speakers for the front L&R (using the same type of tech as the TV, and similar sound characteristics), it worked out 'pretty alright'. I'm sure the tonal mismatch was still very apparent, even with EQ correction (to attentive ears).