To my knowledge, nobody has ever claimed that there should be “one ontology to rule them all.” Instead, what is regularly promoted is ontology reuse and/or integration. For example, the FOAF ontology is widely used in the semantic web to describe persons; why create your own ontology when you can reuse a well-established one? Integration of ontologies allows for conciliation of perspectives, causing data that use these ontologies to become meaningfully related. Admittedly, there are some rather large, comprehensive ontologies out there, and there are some very popular and pervasive ones, too. However, there is no standard or recommendation that requires publishers of RDF data to comply with any particular ontology. You could even ignore the RDF vocabulary if you so please (yes, even rdf:type).

The primary purpose of an ontology (in my view) is to attach explicit semantics to your data. Just as the participant had stated (although he meant it in contrast to the semantic web), there are many ontologies. They compete in the ecosystem of the World Wide Web and evolve accordingly (or become extinct).