The issue the author has with Warren, et al, is not that they don't practice sola scriptura, but rather, they don't practice it the way the author believes it should be practiced. Indeed, the author seems to adhere to the relatively recent invention of solo scriptura (a term, while arguably grammatically incorrect, coined by Protestants themselves, to describe the quandary they find themselves in today, to whit: the practice of using only the Bible to form doctrine, and no outside sources such as history, or traditional practices, OR accepting traditions and church leadership as long as they don't "violate Scripture"). This can be shown by a simple rewording of the author's statement above
If sola scriptura were the formal principle in Warren’s theology tradition of celebrating Christmas, then he would provide vigorous, Biblical analysis using sound exegesis to ground his reformation practice of celebrating Christmas on the authority of Scripture. But his teachings and public statements are not characterized by sound Biblical exegesis.
As we can see here, the author's statement can be used to "justify" rejecting all sorts of "extra Biblical practices", thus, falls under the category of solo scriptura. Again, the author is basically complaining that Warren et. al. don't have the same extra Biblical practices as he does. A complaint that ultimately falls flat on its face if its applied equally, fairly, to reject all extra Biblical practices and or traditions. The only counter argument to that is "Christmas is an old tradition, but Warren's practices aren't old, therefore shouldn't be equivocated to "traditions" like Christmas". A ludicrous rebuttal for what should be obvious reasons.
This is why, ultimately, I've rejected Protestantism in general. There seems to be no consensus about what the fundamental dogma of Protestantism (sola scriptura) means. Some believe as this author (apparently) believes, that "Anything not soundly shown in a positive state in Scripture should be rejected" (which is really solo scriptura), while others believe that there's a place for tradition and church leadership, but they are both subject to, what is eventually, personal interpretation of Scripture. (which is historic sola scriptura)
Why mess with a version of Christianity that can't even agree on their central dogma? I've been told that there's just as much dispute among Catholics/Orthodox as there is among Protestants, but I've found no evidence that there are disputes on dogma, like there are in the tens (if not 100's or 1000's) of Protestant denominations.
That’s strange, I don’t know why some of my post above appears in red font. It wasn’t intentional, for the record.