"This is a t-shirt."
the t in lèine-t is doing exactly what it does in t-shirt - just refers to the shape of the garment. Missing it out in seo lèine-t has the same effect as if you missed it out in 'this is a t-shirt' - i.e. turns it into 'this is a shirt'. Presumably having seo lèine accepted is just a Duolingo glitch.