Their being not-self does not say anything about the nature of their existence, though. It simply tells us that things are not me, not mine. Conversely, it matters not whether things are "inherently existent" or " exist from their own side" or not. Either way, the Buddha's teaching that they are "not me, not mine". "Me" and "Mine" are the source of dukkha here, not some irrelevant speculative equivocation of "inherent existence - or -not".