In what sense? In terms of additional peerings wasting bandwidth proportional to the number of messages? That can be addressed with techniques like erlay ( https://arxiv.org/abs/1905.10518 ) though the erlay paper's settings are tuned for lower latency than something like Bitmessage would want.

In terms of the fact that it is just a global broadcast? Not everything needs to "scale" to handling all messaging for the world. Bitmessage style broadcast can support occasional small messaging for tens to hundreds of thousands of users (maybe even millions) without using outrageous amounts of bandwidth.

Not everything needs to be or should be the next twitter replacement.

The point kinda was to optimize for security over scalability. It wasn't the goal. Go make a messaging system on TOR if you want a less secure more scalable tradeoff point.