As a non owner of the copyright, but beneficiary of a GPL license, you would be allowed to make a derived work (your own "pro" version), and to distribute it (to sell it), but it would have to be under the GPL.
In this case, since it is a library under the LGPL, you could make a client product using this library, instead of a derived work, and you could distribute this product under your own terms, but if you provide the library along, you must of course keep this library under the LGPL (and provide the sources of the library).
So with the LGPL, you are free to make competiting proprietary products.
In this case, since it is a library under the LGPL, you could make a client product using this library, instead of a derived work, and you could distribute this product under your own terms, but if you provide the library along, you must of course keep this library under the LGPL (and provide the sources of the library).
So with the LGPL, you are free to make competiting proprietary products.