The plain English and culinary distinction of fruit vs. vegetable vs. starch is different from the botanical distinction of leaf vs. root vs. fruit (etc.)

Knowing when to use plain English vs. terms of art is something pretty much any scientist learns very early; correcting people is the sign of an amateur or know-it-all rather than a professional.

(If you feel like one-upping them instead, you can point out that not only is a tomato a fruit, it's a berry -- while a strawberry is in fact NOT a berry, but rather an "aggregate accessory fruit".)