This is not a great argument when talking about a Free Software project. Flutter is 3-clause BSD licensed, so even if Google abandons it, it's not like it suddenly traps all the applications built on top of it. If the toolkit code was unusually difficult to maintain, then sure, you shouldn't use it.
The article seems to hint that Google might open-core Flutter like they did with Android, but I see no evidence for this. Hell, even Chrome, technically an open-core project, is more or less entirely usable as Chromium (the Free part of Chrome) and projects exist specifically to remove all remaining Google services and telemetry (e.g. Ungoogled Chromium). If Google made Flutter practically unusable without other proprietary components, then yes, Google abandoning it does become a risk of using Flutter. However, I don't believe this to be the case.
Applications looking out-of-place on Linux is not something Google invented. Linux does not have a common look-and-feel, nor is there a single point for theming to occur. Even concepts like the window decoration, once exclusively the property of reparenting window managers, have been totally encroached upon by the window toolkit. So adding Yet Another Toolkit to Linux app development is not any more of a harm than the current situation already may be.