While that is true, a fact that increases the credibility of the published results is that the study also obtained a negative result.

Besides the multivitamin supplement, they also tested cocoa extract, for which similar effects had been claimed, and for that they did not find any effect.

The fact that the multivitamin supplement had effect is plausible. Presumably a significant number of the participants did not eat a perfect diet that would supply all vitamins in adequate quantities, so for those, taking a multivitamin supplement compensated whatever vitamin deficiency their diet might have had.

Such a positive result does not demonstrate that you need to buy a vitamin supplement, it just demonstrates that it is desirable to eat healthy food. However, there are circumstances when a vitamin supplement may be cheaper or more convenient than buying and eating enough food of an appropriate type.

For example, I take a vitamin supplement from time to time, but that is only because I have a sedentary lifestyle, working at a computer, so I must eat relatively little, otherwise I would gain weight immediately. When you eat little, it is difficult to compose a menu that will provide enough vitamins without also providing too much energy.