I have felled a couple of old apple trees in my garden with a chainsaw after watching a "few Youtube videos".
The trunks were maybe 30 to 40cm in diameter so obviously much smaller trees than that sycamore (well apple trees for a start and much younger) and I can tell you that this is one of those things that looks easy when you watch but not so much when you do it. Handling a chainsaw to be able to make clean, straight cut through takes practice and you need a good chainsaw as well.
So IMHO there is no chance in hell that the sycamore was felled by an amateur.
I believe that one has to be at least 16 to become a trainee so maybe that 16 year old has received previous training and/or his family works in the field. Or he didn't do it himself.
But! It also looks pretty clean for a "first try" so maybe the paint is something else.