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It is possible the article referred to a specific amp/speaker comination that was tuned, or just matched to work together. Most crossovers already have low pass filters on woofers and midrange, so that may not have been a concern, and as you say, some tweeters may not need the filter at all.
On the other hand, I suspect you're correct, and these amps probably do have some low pass output filter. The filters would make the amps safer, and more predictable, and there is probably no good reason against having them.