Or maybe three things we already knew
First

Aubameyang is a lethal striker and will be Arsenal’s first choice.  Three goals in the first 16 minutes is impressive no matter the competition.  Lacazette is a top notch second striker off the bench who will hopefully keep clipping at Aubameyang’s heels and keep him honest, but for now Lacazette will play second fiddle.
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Second

Arsenal has talent in the youth academy.  In the Premier League every single point counts, making it difficult to give young players opportunities.  Lost points at any time in the season could make the difference between a Champions League spot and the funds that come with it, and playing young unproven players can be a real gamble.  It is in the preseason and cup ties where they often find their chances, but these performances cannot always be taken at face value given the level of the competition.  Still, these preseason games will be important for Emery to fully assess his young players and decide which will be ready for a call up to the senior squad during the regular season.  Emile Rowe smith, Reiss Nelson, and Eddie Nketiah all had good showings today.

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Third

Unai Emery will bring a fresh rebirth to Arsenal.  He will bring new tactics, new dynamics, new leadership, new players, and as mentioned make important decisions as to which youth players will come into the squad.  Will he be a David Moyes or a Jurgen Klopp remains to be seen, but he is off to a good start.

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Having said all this, Arsenal played Boreham Wood today, a team in the fifth tier of English Football.  It was a safe opponent for Emery’s first game in charge, but it will be much different come August 12 when he starts his Premier League account against defending champions Manchester City.  Nonetheless, it was a good result and the optimism is welcomed as the last two seasons have seen the club slide away from the top four and a cloud of pessimism has settled over Emirates Stadium.  At the end of the day we cannot take much from a big win against a minnow, but it is certainly more reassuring than a tie or narrow victory.