2 things: 1. coming "to the defense" would be crossing a line on that country's own sovereignty, or its own development/buildup of sovereignty. (China's not interested in the soviet model, and I think this has merit because we saw how areas propped up by soviets fell as the ussr waned and then collapsed.) building infrastructure doesnt mean unilateral alliance, it's a business transaction, albeit one that meaningfully materially strengthens the beneficiary country's ability towards economic development and sovereignty.

2/ someone come correct me if I've got my understanding of how bonds work/macroeconomics/monetary policy/forex backwards but China owns a lot of US debt, and is actively shedding it. Used to be #1 foreign holder of US treasuries, now it's #2 (#1 currently Japan). In the past China bought US bonds using its trade surplus in dollars, which would basically recycle $ to continue to develop its own manufacturing and towards growing its own middle class+its own domestic market. Now that China's developed a pretty robust domestic market (eg doesnt need to rely on US to consume those produced goods and fuel economy), China doesn't need to put dollars back into that machine, but it still has a trade surplus in dollars. So financing other projects like those on BRI or among BRICS with those dollars is a solution to that problem (holding onto that surplus isn't economically sound..), and bonus points for building material foundations for dedollarization.

someone else's analysis that's similar to my second point, and probably has a better understanding of that economics stuff: How China recycles its huge trade surplus with EU, US into BRICS infrastructure projects, risk-free