I'm not sure what that Wikipedia page is citing, but its inclusion of 'among nobles as well' seems to be a little off, especially for the latter half of the seventeenth century. To answer your question simply, they tended to be quite well educated. Still, it's wise not to confuse education as a marker of elite reading, let alone writing, skills; likewise, education and literacy are inevitably linked, but it's easier to focus on the latter for the sake of brevity. As Furet and Ozouf observed (Reading and Writing), “literacy is not schooling, and the history of the school by itself will not suffice to give a full account of the literacy process”. I'll start by providing some general context with some wider remarks about late 17th-C/18th-C literacy.

While total literacy increased from a rate of around 10% of the population in 1650, even by the 1780s, only around 1/8 women could read; in Carla Hesse’s words, “Illiteracy was a distinctly gendered phenomenon by the end of the Old Regime”, where “Most French men were literate and most French women were not" (The Other Enlightenment, p. 9). And as Daniel Roche writes in France in the Enlightenment (p. 659), taking Paris as his scope: “By the end of the seventeenth century, people in the upper and middle strata of society could sign these documents. Overall, under Louis XIV, 85 percent of men signed their wills and 60 percent of women”, though this blossomed in the following century: “under Louis XVI the comparable figures were 90 and 80 percent, respectively”, with literacy increasing “more rapidly among women than among men, and more rapidly in the cities than in the countryside” (Roche, p. 527). Average literacy itself stood at 38.6% (men) and 12.5% (women) between 1671-1720 (Roche, People of Paris, p. 202, fn. 17). But in any case, these also represent wage-earners, not just the elite, and there are source difficulties in extracting literacy from signatures (discussions on this are ubiquitous; e.g. Roche, People of Paris, roughly pp. 200-206; Melton, Rise of the Public in Enlightenment Europe, pp. 81-92). See also this blog post (amongst many others: http://early-moderneurope.blogspot.com.au/2009/09/problem-of-literacy.html) on the impossibility of simple binaries of illiterate and literate: rather, there's a scale, such as with Carlo Cipolla’s idea of semi-literacy (see Literacy and Development in the West, 1969). Moreover, although literacy was rising amongst servants too — one abbé already noted at the end of the 17th century that the “petty bourgeois” would “not employ a lackey … who cannot read or write” (Kaiser, “The Public Sphere”, in Doyle (ed.), Oxford Handbook of the Ancien Régime, p. 417) — the overall figures reduce when you step back to consider all of France, with 29% of men and 14% of women able to sign for marriage registers in the late 1680s, versus 48% and 27% by the late 1780s (Melton, p. 82).

So, in short, the author of that sentence isn't necessarily wrong. Again, I stress that I'm not a seventeenth-century specialist of elite culture, but I do imagine that most would have a good grasp of reading. Where I think the parenthetical assertion does have validity is that writing ability isn't necessarily consonant with reading ability as mentioned earlier, and even then, writing even among the elites would have had orthographical variations, with language reforms in their jeunesse. Furet and Ozouf have already pointed out (Reading and Writing, p. 77) that whereas “reading belongs to the domain of religion and morality, writing is an apprenticeship in utility”. But perhaps others could add on this.