> IMHO desire to prevent learning from your open source code seems kind of irrational and antithetical to open source ideas
You may be right that this is antithetical to "open source" ideas, as Tim O'Reilly would've defined it - a la MIT/BSD/&c., but it's very much in line with copyleft ideas as RMS would've defined it - a la GPL/EUPL/&c. - which is what's being explicitly discussed in this article.
The two are not the same: "open source" is about widespread "open" use of source code, copyleft is much more discerning and aims to carefully temper reuse in such a way that prioritises end user liberty.
You may be right that this is antithetical to "open source" ideas, as Tim O'Reilly would've defined it - a la MIT/BSD/&c., but it's very much in line with copyleft ideas as RMS would've defined it - a la GPL/EUPL/&c. - which is what's being explicitly discussed in this article.
The two are not the same: "open source" is about widespread "open" use of source code, copyleft is much more discerning and aims to carefully temper reuse in such a way that prioritises end user liberty.