they aren’t wrong. backwards compatibility is a suppose to one of the first promises any mature programming languages. unless you make it explicit via noting breaking changes in major version updates (1.X.X —> 2.X.X) or the language is purely for R&D and makes no guarantee of anything
The website says, "EARLY ALPHA Vapour is extremely young, the syntax might change, things will break, expect bugs."
What part of this is giving any sense of stability? It's clearly an experimental language, so I find it hard to understand why you are discussing stability and compatibility at all.
Bugs are normal software development.
Changing syntax and breaking things make work for everyone else for the convenience of developers. Reliability is what makes a tool a tool.