The definition of "boutique" implies a small, exclusive business that specializes on their specific product or style of products and that offers customized service.
Any small business specializing in making speakers should meet the definition of "boutique" but few of those would get many (if any) reviews in publications such as Stereophile. However, I suspect there is latitude with respect to the definition of "small."
Other related terms we sometimes hear are "bespoke" which means the product is custom-made for the buyer, artisanal which implies a distinctive product made by an "artisan," typically in small quantities, and usually by hand crafting and/or by using traditional methods, and "craftsman" which is a person who is skilled in a particular craft. In our hi-fi world, it is not uncommon for boutique businesses to be run by an artisan who makes bespoke products for their clients/buyers.
Other related terms we sometimes hear are "bespoke" which means the product is custom-made for the buyer, artisanal which implies a distinctive product made by an "artisan," typically in small quantities, and usually by hand crafting and/or by using traditional methods, and "craftsman" which is a person who is skilled in a particular craft. In our hi-fi world, it is not uncommon for boutique businesses to be run by an artisan who makes bespoke products for their clients/buyers.