After some more research, I have made some progress towards an answer. Here is the key info of what we have, in case it can help others.
The surface of the aluminium enclosure, or any other uncoated aluminium part, is oxidised by atmospheric air, always ending up with a thin (a few nanometers) layer of aluminium oxide. This layer is an electrical insulator, but since it is extremely thin, it will be scratched off easily when trying to measure its resistance with a probe or when applying several volts.
Yet, it can be a problem for long-term good earthing.
This problem is obviously very important in aviation, where aluminium is often used and electrical earthing is important.
The suppliers of earthing studs (for aviation and other industries) have come up with a solution: aluminium studs are coated with a passivation layer (e.g., titanium) that protects against oxidation but is highly conductive. This is the key bit of info.
Aluminium earthing studs (with a titanium coating) are welded to aluminium enclosures, then nuts with washers and hook-ended wires can be tightened to the stud making the connection to earth.
By the way, we have also learnt that only aluminium welds to aluminium. This is why I am mentioning only titanium-coated aluminium studs when discussing ways to connect to earth an aluminium enclosure.
PS: There is of course another possibility: to simply use a through-hole, nut and bolt (and washer). No welding. In this case, the oxidation of the aluminium enclosure (in particular the area below the nut) could lead to high resistance. It seems that welding would be the best solution.