WP:IAR was Wikipedia's first rule, so it is true in a limited historical sense that all other policies and guidelines have followed it. But it is also true that all Wikipedia policies and guidelines follow from IAR, for the following reasons.
Summary: In principle, a wiki is a fluid system for collaboration without any rules, including any rules against rules. IAR represents the necessary first step in harmonizing this openness with a purpose like Wikipedia's. All other rules are exceptions to this general rule of free, ungoverned collaboration. But an exception to IAR is also an application of IAR. So, more precisely, these rules are derived from IAR. For example, because an open and free encyclopedia that anyone can edit must have some way for contributors to check each other's work, it is necessary to ignore IAR in this regard and require that all information be verifiable. Likewise it is necessary to require that the content on Wikipedia be fundamentally encyclopedic. Some constraints likewise arise from Wikipedia's specific practical and legal constraints, which make it necessary to IAR to establish policies like BLP in order to keep the project viable as a practical matter. All policies and guidelines are valid only to the extent that they are defensible as invocations of IAR against itself.
Another way of stating this is that all Wikipedia policies and guidelines are a means to an end. They are an exception to the wiki's general rule of fluidity in service of our common purpose. That state of exception should be maintained only for as long as it is needed. In the case of core policies, the exception is approximately co-extensive with the project itself -- but any particular formulation of policy should only be applied in such a way, and to such an extent, that it does not run counter to the purpose of the project.