Da, there are conditions upon which brisker roshei yeshiva were historically very maikil with tefilah betzibur; rav avrohom yehoshua doing so under those circumstances is not a surprise to me at all.
One of the rabbonim from the Brog family, i forget which one, gave a drasha about the importance of having a rebbe to us bochurim in camp about 8 years ago. He mentioned his literal adventures with rav dovid soloveitchik, who he developed a close relationship with. He offered to take rav dovid on a bein hazmanim trip, which rav dovid surprisingly agreed to! When it became time to daven mincha, he chose to daven alone, because he was makpid to wear talis and tefilin by mincha, and he didn’t want people around him to think he was a member of a certain chasidus that I’d rather not mention.
Rav chaim brisker held that minyan is not a chiyuv, which is part of why brisker roshei yeshiva aren’t as makpid on it as other segments of the Torah world.
Respecting the position of rosh yeshiva when talking to impressionable bochurim is extremely important. It’s like how parents must respect their children’s rebbeim even if they think that they’re mistaken, because of the position they occupy. Now you can switch your kid’s rebbe, but as long as he’s in the class, he needs to have a high impression of the rebbe, as representative of Torah. For a rebbe in darchei to publicly disparage noy just a rosh Yeshiva, but someone many of his talmidim will go to to learn from, is against his duties as a rebbe. To me, it’s enough to make him upgefreight; disqualifying.
Moshe was even told to show kovod malchus to paroh….is rav avrohom yehoshua worse??
Again, brisker “bikores” is misunderstood, the same way your rebbe misunderstood davening beyechidus. Maybe he should learn the brisker toros before talking about things he clearly has no idea about.