I'm kind of puzzled about the cracks about MPT being

1) discredited, and
2) from the 1970s

Is it possible the authors were referring not to MPT but to CAPM? Even CAPM isn't from the 1970s (it's from the 1960s), but it's closer and it is largely debunked.

If the authors really meant MPT, I'd like citations for when it was discredited, because I seem to have missed them. Are there CFA publications on this? Seems like they would be worth publicizing.

Any time you use an efficient frontier you're using MPT. If you're concerned about portfolios being too diversified, MPT doesn't prevent you from concentrating your portfolio - so long as you don't use constraints, the farthest point to the right on the efficient frontier is either a single security or a very concentrated portfolio.

I realize this isn't the FAJ, but still, I am surprised to read this post in a CFA Institute blog.