。
She testified that the statement was true.
Literal
She [topic-は] that statement [subject-が] truth [copula-である] [quotative-と] testified.
陳述 ('statement, deposition') is legal vocabulary — used in courtrooms and depositions for formal sworn statements, distinct from the everyday 発言. 証言する ('to testify') is similarly courtroom-flavored. The structure ~が真実であると証言する uses である (formal copula) to attach the predicate 真実 ('truth') to its subject before the quotative と embeds it under the testimony verb. The whole sentence is an exemplar of legal Japanese — stiff, formulaic, and unmistakable from a few key words.