I think the line "Their taste was strikingly alike" is brilliant. The word 'strikingly' draws attention to itself. Their taste was so similar that it would strike people as being remarkable how similar it was. Perhaps even too good to be true.

It's deceptive, because the word 'strikingly' was previously used to describe how much Marianne and her mother are alike (ch. 1 ): "The resemblance between her and her mother was strikingly great." And from what we've seen so far, Marianne and her mother do, indeed, seem to be very much alike.

So that word can make you overlook other things we learn about Willoughby. You mentioned that Willoughby went along with Marianne in her expression of taste, even if there seemed to be a difference of opinion to start. I also notice this part of that same passage: "...any young man of five-and-twenty must have been insensible indeed, not to become an immediate convert to the excellence of such works, however disregarded before."