This is patently false. Local indexing and search will always outperform a network call
Of course there are people who are much more popular than me, so it might be possible and I am not aware of it. :)
https://t.me/durovschat Has 9500 members
https://t.me/swhkdemocracy Has 6000 members
https://t.me/linux_group Has 6000 members
https://t.me/PublicTestGroup Has 18000 members
There is no limit to joining these chats. These design decisions Telegram has picked has allowed the app to be quasi social media, rather than just IM. You can't index these and maintain chat history locally. You'd need way more bandwidth and energy and computational power than the average cellphone has.