For further fire on the speculation: Japanese nouns are always both singular and plural. Japanese as a language does not encode plurality. Thus if one considers Lexus to be a Japanese word the plural form being Lexus is reasonable.

Like how tsunami is never tsunamis. Or sushi is never sushis. One might argue that "eating sushi" implies multiples by the nature of sushi. Yet McDonalds sells French Fries not French Fry. Likewise we eat Potato Chips not Potato Chip. Thus in other words beyond Lexus the english language adopts the Japanese style.