Well, yes… isn’t it?
The problem is less easy than it may look like and amounts to the problem of non-committal understanding. Is it the normal attitude while listening to a speaker or just an exception or an a posteriori withdrawal of belief once one notices that the speaker is in any way non reliable?

In Classical Indian philosophy, Naiyāyika authors should uphold the possibility of a non-committal understanding of sentence-meanings, since they are convinced that cognitions need to be proved to be valid in order to be such and that such validation comes from outside (in the case of testimony, typically out of the reliability of the speaker). Mīmāṃsā authors, by contrast, would claim that belief is withdrawn but that the default understanding of a sentence implies the belief that it states something true.

Now, the problem with the non-committal understanding is that it seemse to have been theorised (and called śābdabodha) as such only in Navya Nyāya. Taber (1996) discusses the possibility of such a hypothesis already in the 10th c. Jayanta, but his arguments are critically analysed in Graheli (2015, forthcoming in Kervan).

By chance I read of a similar discussion in the Port Royal Logique in this post, where the problem is also how to account for the distinction between understanding a sentence and believing that its content is the case. The post is highly recommended!