It's not that you can't learn rigor and critical thinking skills as an English major, it's that it's not all that hard to complete an English degree without learning those things. Hence the word "signals".
At one point during the pursuit of my BA in History, I was reading about 1000 pages of primary and secondary source material per week.
Reading primary source material is comparable to reading mathematical exposition in that you have to proceed slowly and carefully.
Then, I had to analyze the material using the various methods of historical analysis/criticism. You are not going to get an 'A' if you just regurgitate facts. You're also not going to get an 'A' if the analysis/argument is sloppy.
Math didn't really get that hard until the proof-based courses, and math degrees usually require only two of those (though I did four). The “computation courses” just consisted of exercises that are only difficult until you discover a small set of tricks.
Now, I'm not sure about what it takes to be a very successful English major, but my anecdotal evidence indicates that the high achievers are pretty smart: One did a math double major, one went from being a LaTex whiz to handling most of the IT at a mathematics journal, and the other is a better-than-average DBA.