Because the branch asks about the nature of the knowledge, not the quality of the employee. A letter describing someone as experienced, senior and valued describes a good worker. The requirement concerns knowledge of the organization's own product, processes or procedures, or an advanced level of knowledge of its processes.
Redirect the letter from praise to description
Employers write these letters the way they write references, because that is the genre they know. The most useful thing an employee can do is supply the questions the letter should answer, in writing, before anyone starts drafting. What is the system, what does the employee know about it that a newly hired professional would not, how was that knowledge built and over what period, who else has it, and what would replacing it involve. A manager given those questions will usually produce a far better document in less time than one asked to write a supporting letter with no guidance. The redirection is not a criticism of the employer; it is the difference between a letter about a person and a letter about knowledge.
Hypothetical example: Olena supports a proprietary beverage-formulation platform, but her manager initially describes her as simply excellent at quality control. An opening assessment asks for the internal technique, the employee's unusual knowledge of it, and the reason ordinary specialists could not readily supply it. Project records, access logs, internal manuals, and training history settle that question. Draft the knowledge explanation before collecting generic references. Cases weaken when the employer relies on praise, seniority, or a shortage of staff instead of describing company-specific expertise. The overseas employment record must also show the qualifying year during the relevant period; specialized knowledge does not replace that separate condition.