No. Seniority, a strong degree and years of experience describe a skilled engineer, not specialized knowledge. The test is special knowledge of the company's own product and its application, or advanced knowledge of its processes, which must be shown by explaining the product and the engineer's particular knowledge of it.
Skill belongs to the person; specialized knowledge belongs to the product
Hypothetical example: the company's first draft support letter describes the engineer as a senior member of the team with eight years' experience and a master's degree, and concludes that she is therefore essential. That letter would likely fail, because it never says what she knows that is particular to the company. A better letter explains that the frost-alert product depends on calibration models the company developed internally, that those models incorporate design decisions not published anywhere, that the engineer authored substantial parts of them, and that adapting them to Oregon microclimates requires knowing why those decisions were made.
It then states what a newly hired sensor engineer in Oregon would need, perhaps a year of internal training, to reach the same capability. The company should resist inflating the claim to uniqueness; the standard is knowledge that is special or advanced relative to the company's other staff and the general labour market, not knowledge nobody else on earth could acquire.