It's certainly an indicator. But, just like airplane orders, the backlog is long enough that you have to put orders in on a speculative basis.

If everything goes as planed, we'll need new equipment in a couple years, so order now. If things don't work out as planned, you might be able to sell your slot, or push it back a few months, or cancel it.

In a frothy market, you don't really know from order backlogs how many orders are based on firm plans and how many are we might want to build chips, so let's order a machine and see if we want it when it comes up. Or the middle ground of we definitely need these machines in the next ten year, and if sales are still strong, sooner is better.

Companies buying those machines need years to get their money back. If they didn't believe they would make a good ROI they wouldn't buy as much.

Presumably it indicates confidence from chip makers in long-lasting demand. With a signal with actual stakes, not just words in a shareholder meeting

It means companies are expecting demand for AI chips to keep increasing