"Vegetable" is a culinary term. It does not mean "plant", it is not the opposite of the botanical "fruit". It means "We use this in culinary traditions similarly to other vegetables".

Pumpkin, Squash and Mushrooms all fit into soup and not into fruit salads, so they're all vegetables. Cucumbers are veggies for fitting into actual salads, though they're only like a few good decades of selective breeding away from being full culinary fruits. These are not exact definitions, but, like most things in life, messy definitions are often the more useful ones.

Since "vegetable" only has a definition as a culinary term, I really don't get why people get so hung up on it. It's not like "nut" or "berry", whose culinary a botanical definitions couldn't be in more of a disagreement.