Corner office is a term that indicates a top management position within a company, and is therefore given to the senior most executives, like CEO, CTO, CFO etc.
Read up about "corner office" here:
A corner office is an office that is located in the corner of a
building. Corner offices are considered desirable because they have
windows on two exterior walls, as opposed to a typical office with
only one window or none at all (windowless offices occupying a corner
of a building are therefore not typically considered “corner
offices”). As corner offices are often given to the most senior
executives, the term primarily refers to top management positions,
such as the chief executive officer (CEO),1 chief financial officer
(CFO), chief operating officer (COO), chief marketing officer (CMO),
chief information officer (CIO), General counsel (GC) and the director
of human resources. In organizations which do not use this corporate
hierarchy, such as law firms and political parties, the corner office
generally refers to the most senior partners or officials who are
involved with corporate governance.
In the example you've given, it means that even though David MacDonald (a CEO) is an ambitious executive, he is also down-to-earth and doesn't want to acquire the top spot for himself.