It is common that very poor countries also have very low unemployment rates.
A typical reason for this is that a large share of the population can always engage in subsistence agriculture, in which case they are both employed and also poor. However, despite the presence of near-subsistence agriculture in some regions, Mexico is a middle income country and not a "very poor" country, so without looking deeply into the data I would not assume this to explain even close to all of the observation.
It could also be related to a) how poverty is defined and the effects that income inequality can have in the context of the specific definition (e.g., a large share of the population earning less than 1/2 of mean or median income, but still able to access basic housing, nutrition, communications, education and health services), b) the minimum wage, for example if it is very low you might have a lot of people working but at a very low wage (consider this in an ideologically neutral sense, since even if literate this does not necessarily imply that skills matching in the labour market can "handle" a higher minimum wage), c) whether a national poverty rate is applied to areas where housing expectations are met at a much lower cost compared to the capital city (or any other major cost differentials that are not adjusted for due to using a national poverty line instead of adjusting for regional cost variations), ...
Also, consider that the stats are usually for market income, without imputing the value of public services such as health and education. This is among some variety of reasons to prefer consumption indicators of wellbeing instead of market income, despite the shortcoming that there is always some subjective element in the evaluation.