Again, legally that's correct. But it goes completely against the spirit of open source and especially the GPL which says in the license itself that "our General Public Licenses are intended to guarantee your freedom to share and change all versions of a program". If you can't run a modified version of a program without getting sued, you practically speaking do not have the freedom to modify it.

Elsewhere, the GNU explains why this is important[1]:

> With proprietary software, the program controls the users, and some other entity (the developer or “owner”) controls the program. So the proprietary program gives its developer power over its users. That is unjust in itself; moreover, it tempts the developer to mistreat the users in other ways.

> [...]

> Freedom means having control over your own life. If you use a program to carry out activities in your life, your freedom depends on your having control over the program. You deserve to have control over the programs you use, and all the more so when you use them for something important in your life.

Telling your users they can't run modified versions of your open source client goes against this principle.

Again, I'm not necessarily saying Bambu isn't within their legal rights to do this, I'm just saying it's a jerk move.

[1]: https://www.gnu.org/philosophy/free-software-even-more-impor...