Yes, preseason matches sometimes don't provide a clear picture of a team's true performance. Last season, I often saw teams perform poorly in preseason but then perform well when the season started. And vice versa. So, we can't even judge a team's overall ability based on preseason alone.
However, for teams with several new players, the coach will usually test them by placing them in different positions, where they will then assess their suitability. And sometimes, new players don't perform as well as expected, as is the case with Gyokeres at Arsenal right now. But again, this is just preseason, and what we'll truly see is when the season actually starts.
In the match against Tottenham, Gyokeres entered the field only in the 77th minute, and against Villarreal he played from the beginning of the match but was replaced in the 63rd minute, so I think it is difficult to understand how good he is in such a short time. He needs time, and in the first rounds it will be clear how ready he is to play and how much Arteta will trust him with a place on the field.