studies have shown that immigrants from Latin Ameraica learn English after the first generation. Also data suggests that more than 2/3s who immigrate to the US in the past 10 years from Latin America did it illegally. Therefore putting 2 and 2 together, anyone speaking spanish as a primary language has a higher probability of being a new arrival, and if they are a new arrival there is a high probability they are are illegal.
"Speaking Spanish doesn't mean they were Mexican."
You are splitting hairs. In this situation it was very obvious that they were recent immigrants from south of the boarder. I can not say if they were illegal.
This was not just a couple, but 20 or more very tan men and women.
Memphis has never been noted as a settling area for immigrants of any nationality, but there are small groups of Brazilians, and other nationalities that have adapted to American culture, not Mexican.