Lovely chick , congrats. The earliest I've had chicks hatch is 18 days n these were my own eggs that I collected and wrote the date on at time of collection.
I collect all the eggs here twice daily every day and as my breeding birds are kept in smaller coops I know I'd not missed any eggs. Our temps here aren't that high here being Ireland even though I'm right down in the South West, so I can only think the hen must have sat on it all day n started off the eggs. Anyhow the chicks hatched happy n healthy and are now running around with the other youngsters causing chaos.
Good luck with the rest of the hatch.