Enticing users to blindly run remote 3rd party code on their machines is IMHO not a proper thing to do.
This approach creates a dangerous mindset when it comes to security and good practices in general.
Installing via package managers or installers also runs remote 3rd party code on your machine, so I don't see much difference from a security perspective. You should make sure you trust the source before installing anything.
Even if we skip a step ahead and consider that this script then installs a binary blob... the situation doesn't get any better, does it?
If you find any of this as something normal and acceptable, I can only strongly disagree. Such bad practices should be discouraged.
On the other hand, using a distro's package manager and a set of community approved packages is a far better choice when installing software, security vise. I really don't see how you could compare the two without plainly seeing the difference, from a security perspective.
As an alternative, if the software is not available through a distro's package manager, one should inspect and compile the code. This project provides the instructions to do so, they are just not promoted as a first choice.
I can't help coming to a conclusion, that you've largely made my point about bad practices and having a wrong mindset when it comes to software security.
You can also build from source if you prefer: https://docs.plandex.ai/install/#build-from-source