It can be true. Although any protein can be hydrolyzed, having its peptide chains broken into simpler forms, I would expect a product to specify the source unless there's some reason to be vague.
When I see "hydrolyzed protein" on a label, I assume it's from a low-quality source such as gelatin or collagen. If the protein was from a high quality source such as whey, surely the producer would want to emphasize the source.
When there's so much specific information on labels, I can see only one reason to be vague about a major component such as protein--to conceal information that the producer doesn't want to reveal.