Neville,
Of course it is not pashut, as I have stated each case and subject is an entirely different status. Teffilin on Chol haMoed is an ‘halacha Minhag” I.e. the Bais Yosef etc. And has been accepted by ALL as the Minhag Yerushlayim/EY hundreds of years. Whereas, customs that have no halachik basis and was never accepted or implementer by all is a different story.
In the above mentioned case of Rav Dovid of Karlin is regarding teaching secular subjects/language, wherein there was a Cherem against it..