so 18,83% of supply is basically burn

if that is the case, shall we be happy with it?

supply become smaller and ... may be a good accidential burnt
burn = unusable coins since polo had been lost their priv key

Yes, but we have no proof for this option...
Or do you think they also lost the keys to Ardor, Bytecoin, Gridcoin Research, Steem Dollars, Siacoin and STEEM ? These coins are also locked at Poloniex.