It doesnt say lean to me, but if your rig was running rich at another throttle position that what it was on when it seized you can be fooled by a plug. the piston says cold seized to me too.
Rick Peterson from RPM used to caution people about a cold seizure of a different nature: one where the bike is warmed or heat cycled sitting still, then hopped on and ridden through the gears and the radiator gets a big rush of cold air and the cylinder goes "small". I have several bigbore 125's and have never had that happen and typically i warm them up for about 45 seconds and when they run good off the choke I ride off.
if yours has an airleak it can run lean at the more open throttle settings and still be rich (hence the plug reading) at low settings where the vacuum is greater.
fyi mine all run richer on the stock jetting after the big bore work is done. I have 139's and 144's, yamaha and honda (in the past). i have seen some work done by butcher shops that needed way richer jetting and ran like pigs and seized easily because the proper head work was not done.