The copyright argument is the only relevant argument. If the new work is a derived work of the original, then it follows by definition that the new work is under the copryight of the original's author(s). Since the original chardet was distributed by its author(s) only under the LGPL, any copy/derivative of it that anyone else creates must be distributed only under the LGPL, per the terms of the LGPL.

Now, whether chardet 7.0.0 is a derivative of chardet or not is a matter of copyright law that the LGPL has no say on, and a rather murky ground with not that much case law to rely on behind it. If it's not, the new author is free to distribute chardet 7.0.0 under any license they want, since it is a new work under his copyright.