I don't know how it is in general but Indian philosophers tend to think of their current idea as the 'foremost' idea. Or, perhaps, it is just a linguistic...umm...idiosyncracy. You know, like the 'mother of all battles' etc sort of hyperbole. So, at any point in time, one finds that the foremost virtue is the virtue under discussion only to discover that there is another 'foremost' virtue a couple of sentences down the manuscript. You need to brush it aside much like you brush aside this GOAT (Greatest Of ALL times, in case you have just crawled out from under your rock) reference to cricketers that varies from one IPL match to another.
So, then, when Tiru says this, you need to stop mentally judging him for scattering his 'foremost's across his Kurals. THAT 'foremost' maybe a linguistic style issue but what he is saying is not untrue.
Vinaiththitpam enbadhu oruvan manththitpam matraya ellaam pira - Tirukkural
The foremost strength needed for executing a task is firmness of mind; all else comes a distant second - Loose Translation
Tiru sets aside even talent and knowledge as secondary to a firm and determined mind when it comes to effectively finishing a job. To conceptualise a task, to strategise and plan it, to have the abilities to do what is necessary to carry it out - all of that, in his opinion, is secondary to the will of the person to do it.
AND why would he not consider it so? I mean, have you not met with people with great talent for, say, writing, AND who have honed their craft to perfection but who do not finish a single book. Where would, say, a Stephen King be without his determination that kept him rewriting his first book over and over again?
How many good public policies stay in the files of government because the rulers do not have the will to pursue them to conclusion? The greatest plans and strategies will languish in moldy papers (in decaying chips? Perhaps!) without a determined person who will see to their implementation on the ground. The perseverance to overcome all obstacles, to course-correct where necessary and complete the job would not exist without that firm will. The weak of mind will abandon the job at the first obstacle.
A meticulously planned strategy will be seen as such ONLY when it is implemented. Otherwise it will only be laughed at as Mungerilal ke haseen sapne.
I read something somewhere which encapsulates this very well - 'Vision without implementation is mere hallucination'. AND implementation requires the firmness of will which Tiru sets above all else.