This is very good, thank you Dharma Space for finding this. Always good to find gems like these!
There are similar stories about Trijang Rinpoche also. Both these lamas were very famous throughout the lands of Tibet and their teachings highly sought after even by people of other lineages. From what I understand from reading these old biographies, there was evidence of a lot of harmony between the sects.
Recent allegations of Gelugpas being sectarian comes from a very baffling and strange place. How can the fathers of our lineage act in such harmonious and peaceful ways with all other sects, but now their spiritual children are being accused of being sectarian? How can the single practice of a Dharmapala be sectarian? It is illogical.
What DharmaSpace said is very logical and true, that many are advised not to seek teachings of other sects if our foundation in Dharma is not yet stable. It is for the benefit of the student, that they don't become confused or pulled in too many different directions. This is something that is advised and practiced in all the sects. I am sure Sakyas also choose to remain focused and study their own sakya teachings and gain a firm foundation first before taking teachings from other sects; the same too for Kagyus and Nyingmas. This is something similar even in a secular world. People focus on studying any particular discipline with only one or two main teachers / masters and not go to several different places to learn at the same time! It is very much common sense. Saying the gelugpas are sectarian because they don't incorporate other sects' teachings is very illogical and just plain incorrect. In that case, all students of other sects who focus only on their teachers and teachings are also sectarian? It cannot be.
Sectarianism is only when you strive to make your own sect grow AT THE EXPENSE of another sect; or when you put down another sect, destroy another school, defame or slander any other traditions / sects. As far as I know, no Gelugpa school, centre or teacher does that. They may choose to focus only on their teachings, but this is a far cry from doing anything remotely damaging to any other school or practice.