A part of it is horrible practices and a work culture which incentivizes them.
Who can be happy when the code doesn't work half the time, deployments are manual and happen after work hours, and devs are forced to be "on-call"?
Introduce Test-Driven Development, Domain-Driven Design, Continuous Deployment with Feature Flags, Mutation Testing and actual agile practices (as described in the Agile Manifesto, not the pathetic attempt to rebrand waterfall we have in most companies) to the project and see how happiness rises, along with the project's reliability and maintainability.
Oh, and throw in a 4 day work week, because no one can be mentally productive for that long.
IMO the biggest problem in the industry is that most developers have never seen a project actually following best practices and middle management is invested in making sure it never happens.
There are good reasons to dislike Telegram, but having "just" 30 engineers is not one of them. Software development is not a chair factory, more people does not equal more or better quality work as much as 9 women won't give birth to a baby in a month.
Edit:
I don't think fighting legal requests and content moderation is an engineer's job. However, the article can't seem to get it straight whether it's 30 engineers, or 30 staff overall. In the latter case, the context changes dramatically and I don't have the knowledge to tell if 30 staff is enough to deal with legal issues. I would imagine that Telegram would need a small army of lawyers and content moderators for that. Again, not engineers, though.