@Brandish μ
Really, it does not matter, as long as it grab attention. For example, they could chain themselves to a goal in the middle of a soccer match. Really anything that causes flashy disruption but that does not actively harm people. Disruption of the Status Quo IS the aim of this.
There are lots and lots of more rational forms of protest or information. The question that this protest, when arguing about it, can raise is: If you dont like just stop oil, why do you not march with fridays for future or a similar environmental organization that does not use unlawful protest?
Either that points a person who DOES care about climate to do more, or the person arguing has to admit they do not really care, which directly is the reason why that form of irrational protest is happening and will keep happening. Because people are still in denial about the catastrophe that is to come.
As such, this protest is effective in legitimizing lawful forms of protest and action and giving them attention on the long run.
So, if people ask "why harm a painting for climate", answering "because the IPCC report didnt work yet and my protest signs are ignored" is not a bad answer. You need a form of protest that cannot be ignored, no matter what you do to achieve it, as long as you point to the right problem.