The real collapse didn't occur for several years.
They were wrong on the timing, which is fairly common, while being right about the cause and ultimate outcome.
China of 10 or 15 years ago is not China of today, as it pertains to their debt situation. Going from a 130% debt to GDP ratio, to 300% (or higher if you count shadow debt), puts you in a fiscal place that few nations have ever navigated successfully. Now China is reversing course on deleveraging and proceeding to leverage up further, likely making the situation worse.
300% debt to GDP may be sustainable with 8% real growth. How about at 5.5% growth? 4.8%? How about 450% debt to GDP, at 5.5% GDP growth? There are limits to this sort of approach to propping up an economy with very large sums of debt and slowing growth. How much does their growth drop off if they can no longer continue to expand the debt so dramatically? And when that happens, can they still service their debts well? The question is exactly where the limits are and what the consequences will be as they're hit. That varies from country to country, depending on the unique circumstances, and depending on the type of debt. What you can be certain about with China, and what it's doing, is that it's not good. Even if that immense pile of total debt - which continues to grow rapidly - doesn't implode their economy, it is very likely to stagnate it at a minimum by robbing the economy of the growth capital it needs (diverting it to rather incredible debt interest payment sums, as is the case now). As that debt goes higher, more and more of each incremental dollar of GDP growth is going to maintain it.
While all of that is happening: China has large pension funding problems, very high real corporate tax rates that are smothering its economy, a weak social safety net, a shrinking small business market and expanding government-corporate sector that is far less efficient than the private sector, a rapidly aging demographics problem, contracting labor force, and soon to be contracting population. Now juggle all of that with their debt load, before they get rich/affluent at the median, and before they even build out a meaningful social safety net for the bottom billion people (social safety nets & welfare policies being the usual primary cause of large debt accumulation in the developed world).