Basically, that sentence to me means that TEI is a candidate, and nothing else. There are no interesting formats that are isomorphic to TEI; all formats have chosen to handle certain features in a distinct way, don't include some features that TEI does, and often include some features that TEI doesn't.
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You can pass text among formats as long as you provide an unambiguous mapping between equivalent entities and confine your markup to those entities which are mapped. In any case we will need to create a well-defined list of semantic entities and attributes we expect PG to support. As long as those entities are distinct the list can be as extensible as we want. But you are correct, if there is any entity or attribute definition that isn't mapped, information is lost. We can't build a master format on any markup that doesn't support the full set of definitions. There's no existing standard for which we can agree to support everything that particular standard supports. Even within a standard the same semantic construction can be represented in various ways to various degrees of detail.