All genuine historical work is philosophy, unless it is mere ant-industry.
In the quoted sentence, ant-industry is a figurative reference to the work of ants. That point seems clear, but the meaning beyond that point is murky. I give two different interpretations of the sentence in the following paragraphs.
With the antecedent of it taken as genuine historical work, the sentence says that genuine historical work (which presumably is good work) sorts out into two classes: philosophy, and “mere ant-industry”. The latter work, being good, perhaps is careful and scholarly collection and organization of facts, followed by presentation of those facts in dispassionate but lucid prose.
With unless read as a botched substitute for the word else, the sentence says that genuine historical work is backed by philosophical insight or bias, and says that just collecting and organizing facts without some slant or interpretation or overarching concept in mind is busy-work, like that done by mindless ants.
Edit: This edit adds four notes.
Note 1. As FumbleFingers remarks, ant-industry is not in common use, and as TimLymington suggests, it may have been coined for the occasion. In short, ant-industry is not an idiomatic phrase and has no widely-known clear-cut meaning.
Note 2. I haven't looked at the German-language original and it could be the case that the unless/else issue is a translation problem.
Note 3. I think Spengler intended to denigrate historical work that is not based on a philosophy, but if interpreted strictly the quoted sentence doesn't quite work, because of using unless instead of else. It often escapes strict reading, as shown in the following three examples.
Nicholas Gier in Wittgenstein and Phenomenology treats ant-industry as something lowly. Gier quotes the sentence and then writes, “... it is my hope that [this study] succeeds in being more than just ‘ant-industry.’”
In Essays in European History (Ed. J K Burton, C W White) Enno Kraehe quotes the sentence and then seems to imply that ant-industry is equivalent to “establishment historiography”, an abhorrent substance.
In a paean to Spengler's The Decline of the West, Emery Neff in The Poetry of History says, of the sentence, “Curt sentences stress leading contentions.”
Note 4. In its original context, as seen via google books or via archive.org electronic editions, the sentence seems to me still less clear. The sentence after it reads, “But the operations of the systematic philosopher are subject to constant and serious error through his assuming the permanence of his results.” Perhaps Spengler meant that neither philosophy nor ant-industry is enough?