Irrelevance here isn't just economic. It's the fact that US (along with Japan) used to be the main trading partner for ASEAN region for sometime and therefore had a sway in the region for decades. Now, inter ASEAN trade along with China trade has made US less impactful. US can't turn the region against one another and the close trade with China ( along with significant infrastructure development projects China introduces) makes US power less relevant.
If one counts the dollars, then US indeed hasn't lost relevance. But it can't call the shots or play the geopolitics game with much ease as before. A strong Anti War, Anti confrontation, pro stability, pro economy tendency has evolved. I suspect the economic crashes of 1990s of ASEAN had a big role to play here. There are some components, some of which is linked to US. US "forcing" Japan to sign the Plaza Accords and subsequent economic bubble burst and the impact it had on ASEAN countries which depended on Japanese Investments/Credit and of course the IMF swooping down to "help" are some big factors.
Most would know what kind of help does IMF really give. The help from IMF comes with conditions that are often impractical and unsuited for developing countries ( like advocating less government involvement, less state backed enterprises, more capitalism ( without the capital) etc). Members from ASEAN countries here ?
Any case, USA is not relevant as it once had been. Each ASEAN country have their reasons but RCEP will build more economic dependencies and create more reasons to keep some distance with US. China wants ASEAN to be stable so that it can focus elsewhere.
where have you been ?