Glutamate simply enhances flavor, that is, it makes it more intense. It doesn't change flavor. The food listed are ones with high concentrations of naturally occurring glutamate.
More accurate to say that it doesn't have a nice flavour, by itself. In combination with other flavours, it certainly does change the overall. In this way it is somewhat akin to table salt.
That probably explains it. I prefer simple food and don't enjoy intense bursts of flavor. But still, isn't the context of the sentence suppose to mean that those foods are more enjoyable because the higher levels of glutamate that make it more flavorful?
I think the author is writing from the bias that more flavor = more enjoyment, which many people will agree with. There are however, many like yourself who don't find intense flavors to be enjoyable. I would guess that the type of people who would read the Food and Drink section of The Guardian would predominantly be those interested in lots of flavor in their food and so the writer is pandering to them.