It is true that during prayer hours the prayer hall is restricted to the murids of Mawlana Hazar Imam so they can perform their tariqah practices as guided by the Imam. It is a private space designated by the Imam for Ismailis.
However, it would be incorrect to say non-Ismailis are not allowed in Jamatkhana. Under the guidance of Mawlana Hazar Imam, Ismailis have established several ambassadorial buildings (Ismaili Centers, Darkhanas) where in addition to the religious practices, the Jamat engages in social activities that may be of secular nature. For instance, Ismaili Jamatkhana and Center in Sugarland (Texas, US) hosted the Sugarland Mayoral debate on April 20th, 2019, where many non-Ismailis participated. In the UK and many other countries, Ismaili Jamatkhanas participate in annual Open Houses where people of all backgrounds visit public and private buildings to learn about different cultures and communities. Similarly, in other countries, Jamati Institutions organize academic and other trainings where scholars, experts, and people of other faith communities come to the Jamatkhana premises (libraries, REC halls, auditoriums, etc.) to train our Jamati members and participate in different activities.
It is important to recognize that in South Asia and elsewhere, several Jamatkhanas that were built decades ago have been structured in a way that do not allow a separation of the social area and the prayer hall. And considering the deteriorating security situation in some regions, the Jamat feels it is in their best interest that those spaces are restricted to Ismailis only.
For more information, please visit the following links:
Ismaili Jamatkhanas, Congregational Practice and Privacy https://the.ismaili/jamatkhanas-congregational-practice-and-privacy-0
Situating Jamatkhanas within Diverse Muslim Spaces https://the.ismaili/situating-jamatkhanas-within-diverse-muslim-spaces-gathering-and-piety
Karim Jivani, “Muslim Spaces of Piety and Worship,” The Ismaili UK, March 2006 https://prod-static-iis.s3.eu-west-2.amazonaws.com/s3fs-public/muslim_spaces_of_piety_and_worship-karim_jiwani_ed_zl_rh-_november_2017.pdf
Jamatkhanas: Privacy and Purpose https://youtu.be/iYz7rxXMnwY