Sorry, I don’t have Yeshurun.
When he says “pesak hamekubal” I assume he means the accepted halachic practice, and that the Mishnah Berurah didn’t care as much about it.
In a somewhat similar vein, R’ Michael Broyde wrote:
As to whether the Mishnah Berurah was the de facto, go to sefer in Europe, having not lived then, I don’t know. All I can say is that I have never heard anyone who had lived then, say that the Mishnah Berurah was accepted, while I have heard people who had lived then, say that it was not accepted. Additionally, I pointed out earlier that the popularity of a sefer does not necessarily reflect on its halachic acceptance.