No way this is possible (unless they are switching the signal between 3.0 and 1.0). ATSC 3 uses OFDM modulation which 1.0 receivers are blind to regardless of how the individual subs are encoded (MPEG2, H.264, H.265).Isn't that what they're doing with KSTR? Or are they occasionally switching between the two versions (like the WJW ATSC 3.0 test station)? It's not very clear which of their subchannels is using which version according to the Rabbitears listing.
EDIT: Unless they are transmitting in 1.0 modulation (i.e. 8VSB modulation) but encoding some of the subs in H.264 or even H.265 (newer sets might be able to decode at least 264). But this isn't really ATSC 3.0.