Once oxidized, either naturally or by anodizing, aluminium is basically pretty corrosion resistant. It doesn't like salt solutions much, but they still make boats out of it for marine use.

Do you think this is actually a particular type of aluminium, as stainless steel is a particular type of steel, or simply a surqualification (for marketing purposes, generally) along the lines of "organic wood" or "pure spring water" "underwater subsea pipeline" (they generally are organic and pure and under water, respectively)?

Either way (ordinary aluminium or specially alloyed or coated aluminium), "corrosion-resistant aluminium" says it.

Only medium confidence because I really would like to know more.

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Note added at 4 hrs (2010-12-19 23:01:38 GMT)
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My Disagree to another answer has been "corrected" with "Actually, anodizing doesn't 'create' an oxide layer - it's already present and anodizing merely consolidates it, as mentioned in the note added above."

Well, yes and no. True, aluminium straight out of the mill (bright or mill-finish) will soon start to acquire a thin layer of natural aluminium oxide. To obtain a thicker anodized coating of aluminium oxide, this pre-existing layer, which is actually very tough, however thin, has to be broken down to expose the underlying "pure" aluminium and oxidize it further. In addition, in many anodizing processes the alu. is pickled - dipped in acid or solvent solutions - to remove or weaken the thin natural oxide layer first. The electrolytic process involves an acid bath which eats at the pre-existing coating, enabling the electro-chemical component of the process to reach the underlying aluminium and build a thicker layer of aluminium oxide on the surface.

Stainless steel is made "stainless" by alloying it with things like nickel, manganese, and above all chromium. Although pure aluminium, once oxidized, is pretty corrosion resistant, the performance of aluminium can, like steel, be improved by alloying it with other metals. This garden furniture might use one such alloy, such as one of the marine alloys or something like magnalium:

Magnalium is an aluminium alloy with 1.5 to 2% magnesium and small amounts of copper, nickel, and tin.[1] Some alloys, intended for particular uses at the cost of poor corrosion resistance, may consist of up to 50% magnesium. It finds use in engineering and pyrotechnics. Alloys with smaller amounts of magnesium (about 5%) exhibit greater strength, GREATER CORROSION RESISTANCE, and lower density THAN PURE ALUMINIUM. Such alloys are also more workable and easier to weld than pure aluminium.
http://en.wikipedia.org/wiki/Magnalium

Without knowing for certain what feature of the aluminium renders it inoxydable, or even if it is more inoxydable than your run-of-the-mill aluminium, I would advise keeping it general.

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Note added at 15 hrs (2010-12-20 10:17:08 GMT)
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les matériaux très spécifiques à l’instar du teck imputrescible ou de l’aluminium inoxydable

That's it then, we're dealing with "overqualification", like "language translator" or "technical engineer". Teak is by definition non-rotting and aluminium corrosion resistant. It might have been better written:

du teck - imputrescible - ou de l’aluminium - inoxydable
or
du teck, qui est imputrescible, ou de l’aluminium, qui est inoxydable.