It’s just marketing to appeal to buyers who want a sporty car without actually calling it sport,

This reminds me of a TV programmes some years ago about salesmen and their company cars.

One interviewee stated that he had a 1.6i. And the i stood for 'important'.

What a ****!

It's a bit of jargon taken from the phrase 'product line', meaning a more or less related and similarly branded range of products from a manufacturer.

Somebody in marketing somewhere sometime thought it sounded better than "trim level", "grade" etc.

From a marketing point of view, different trim levels are key to the margin mix as a route to "added value". Trim level is a separate dimension to engineering differences and costs relatively little in manufacturing to step up compared with the increments in price that can be charged.

The word "line" has an amazingly large number of definitions, but the one that seems appropriate here is "a defined range" (of products, jobs etc.).

Examples: "The X co. has launched a new line of men's wear."

"What line of work are you in?"

Most 'Sport' models are nothing of the sort and in reality are 'Lux+' models with lower profile tyres (another longer lasting fad) and no difference in the engine spec over a good number of models below it, which I never understand, given a nice smooth ride is part of a car being 'luxurious', or at least it used to be.

Rather like film and TV producers, its all about copying others who appear to be 'trendy', but in reality they mostly are behind that curve by some margin.