I would say this is true and for one line or two lines only. For blocks of text, readability is improved by a mixture of upper and lowercase.
As an example, it is dead easy to spot common speed limit signs, mainly because there are so few speed limits. Even when my sight was failing, I could make out the speed limits at a distance with little trouble. Not because of some magic of the text, but simply from familiarity.
So, in an age where most of what we read is mixed case, it makes intuitive sense that we would be better at reading it. I fully realize intuitive sense doesn't translate to an explanation and is why I'm asking the question.
The Quick Brown Dog Jumped Over The Lazy Fox
-or-
Tha Quiek Brawn Deg Junped Ovar The Lasy Pox
... if you're expecting a certain phase, your brain can error-correct quite well.
Obviously this might not work where letters are important (say a 24-character software license key), but where words or prose is concerned, what matters is the whole.
Of course, depending on font we have issues with all caps as well, especially when mixed with numbers: "Z" and "2", "O" and "0", "Q" and "O" etc.
Typography is not an easy subject.