Perfection is always relative to a purpose or telos. So in the case of artifacts - which are the products of engineering, arts, and techne in general - the purpose isn't inherent to the object in the manner it is in oak trees and human beings, but projected onto the thing. It lives in the mind of the designer and the user. That's why we can say that technology is an "accidental" extension of human beings by which human purposes are realized and mediated in ways that inherent human powers cannot directly accomplish.

So, yes, if you have a purpose in mind, you can build something that can be used for that purpose, more or less effectively. The more general the purpose, the less specific the implementation; the more specific the purpose, the less general the implementation. Over-engineering here is something that indeed goes beyond the purpose of something, with feature creep as an example of moving goalposts and a slippery grasp of the limit.

However, production is only one layer of concern in engineering. You can talk about ideal artifacts whose forms perfectly enable the realization of some purpose, but there is the practicality of constructing one that must also be considered. Think of diminishing returns. Does it makes sense to invest a certain amount of time and resources into producing a more perfect artifact? If not, then pursing such perfection is itself an imperfection on the part of human judgement. That is, a more perfect engineer might know the limit of perfection for an idealized artifact, but he also knows what the perfect practical limit of justifiable effort is. Perfectionism is a sign of an imperfect engineer.

Returning to feature creep, the author might argue that the moving goalposts are a sign that the purpose is being refined, but feature creep can also mean that the shifting purpose is detached from real needs. So, in that case, the purpose served is no longer the purpose that ought to be served, and in that sense, while one may realize an end perfectly in a relative sense, the end pursued is itself the result of imperfect understanding, which is effectively what the author is gesturing at when discussing requirements.