It can be, if the knowledge is of the company's own validation protocol, criteria and tools rather than of assay validation generally. The deciding evidence is the protocol's authorship and revision history, not the scientist's qualifications or how difficult it would be to replace her.
Where general science ends and company knowledge begins
Specialized knowledge means special knowledge of the petitioner's product, service, research, techniques or management, or an advanced level of knowledge of the company's processes and procedures. A degree in microbiology and years of validation experience describe a capable scientist, which is the general labour market. What the petition must isolate is the layer above that: the company-specific acceptance criteria, the internal software templates, the documented rationale for each control, and the way the protocol interlocks with the company's accreditation.
Then it must show that this layer is not readily taught to a new hire in a short period and that the applicant holds it, ideally as author. Avoid claiming that she is irreplaceable; scarcity of staff is not the test. The stronger framing is that the US affiliate cannot adopt the protocol without someone who understands why it was written the way it was, and that she is that person.