No, I wasn't on this forum then. I would be pretty impressed to see such predictions pan out as described. But I would also challenge how many other theories were proposed at the time? Sure, one or six start to emerge as more plausible as more people clamor to refine it.
An educated reader can make almost any theory sound plausible, even if it was not the athor's intent. Ever read the essay which demonstrates that Fight Club was about Calvin and Hobbes? Or any of the more plausible theories from the WMG (Wild Mass Guess) section of tvtropes?
Everyone invovled will remember the correct theory with vigor once it has been proven, regardless how many erroneous theories were proposed alongside. Or how many erroneous details were accidentally attached to the correct theory, though they were part of a different puzzle.
Dalinar-as-Radiant may well prove to be true (likely), this text may even point to it (I have my doubts). I'm just reserving judgment because the passage quoted doesn't prove anything conclusively to me-as-reader without something more substantial.
I'm still not remotely certain why you think the evidence is bad or weak. Two knowledgeable characters, over the course of a few very crowded minutes, notice in passing that Dalinar is doing things that shouldn't be possible, even in the magic system they are already familiar with. It can't be the grandeur of Shards, like you've been trying to claim, for the very simple reason that the viewpoint characters are all familiar with the normal behavior of shards, and it isn't anything they have seen before. (Most observers, even in-world, would probably not have noticed anything out of the ordinary.) They don't remark on them afterwards, because the minutes were so crowded, and it's easy to write things like that off as being due to a heated imagination. They don't expect anything unusual to happen, so they write it off.
We, on the other hand, are readers. We know, from other characters' viewpoints, that the magic is coming back. Kaladin, Shallan, Jasnah, etc., have begun manifesting things that are the stuff of legend. We also know that the Knights Radiant had more powers than "modern" shardbearers manifest, and we even know some of the manifestations, such as the glowing armor, manipulation of forces, and so on. In fact, we even know that one of the orders is
called Stonesinew, which strongly suggests increased stamina, speed, and so on, very much like Thugs from mistborn. In addition, we have the further meta-knowledge that the books are about interesting characters. Combine all those with the fact that, for a very short time, Dalinar exhibited unusual strength, speed, stamina, control over his trajectory when he shouldn't have had it, and even possibly began to glow during this unusual burst of strength (although the viewpoint character writes that off as imagination), it seems
extremely likely that he is manifesting the powers of one of the orders of Knights Radiant. Lastly, we have the fact that magic on Roshar is related to character traits, and that Dalinar has been improving his character enormously from reading Way of Kings, making him more likely than the average Schmoe to be a Radiant, and I have a hard time doubting this theory.