Hmmm may be it is confusing… But how do we know that there isn’t a set of weights that lead to a weighted average vector very close to the [CLS] token?
Finally obviously it is not a simple weighted average since each token’s vector representation is repeatedly influenced (in each encoder layer) leading the final representation (not to mention the passes through FC layers) to be obviously a bit more than a straightforward weighted average but IMHO the idea holds due to how attention is computed.