The comment is incorrect; creating a derivative work without permission is still disallowed, even for private use.
In US copyright law 17USC 106 defines the exclusive rights that the copyright holder has, the right "to do and to authorize". The second of these is:
(2) to prepare derivative works based upon the copyrighted work;
Note that the right is the right to "prepare" a derivative work, not the right to "distribute" or "sell" the work.
US copyright law defines a derivative work in 17 USC 101 which reads:
A “derivative work” is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications, which, as a whole, represent an original work of authorship, is a “derivative work”.
The laws of other countries are similar to US law on this point. Article 2, paragraph 3 of the Berne Copyright Convention provides that:
(3) Translations, adaptations, arrangements of music and other alterations of a literary or artistic work shall be protected as original works without prejudice to the copyright in the original work.
However, it should be noted that if a person creates a derivative work in private, and never shows it to anyone else, the copyright owner would never learn of it, and so could never sue for infringement. But if it were shown or described to anyone, and the owner did learn, then he owner could in theory sue. Whether the owner would choose to sue over a derivative work never circulated is a different matter.
The real effect of this law is that when an infringing derivative work is distributed and the owner wants to sue, the owner need not prove distribution. Proving creation of the derivative work is enough.
The quoted comment asks about whether such a rule is "unconstitutional or something" and says that "You should be allowed to do whatever you want with your own stuff in your own home."
The US constitution does not grant any such broad right. There are lots of things one might do in own's own home that are illegal: building a bomb for example.
Article I, Section 8, Clause 8 of the US Constitution, sometimes called the Copyright Clause or the IP clause, grants Congress the power:
To promote the progress of science and useful arts, by securing for limited times to authors and inventors the exclusive right to their respective writings and discoveries.
See the LII page "Intellectual Property Clause" and the page Nature and Scope of the Right Secured for Copyright where it is written that:
Congress was within its powers in giving to authors the exclusive right to dramatize any of their works. Even as applied to pantomime dramatization by means of silent motion pictures, the act was sustained against the objection that it extended the copyright to ideas rather than to the words in which they were clothed. {Kalem Co. v. Harper Bros., 222 U.S. 55 (1911). For other problems arising because of technological and electronic advancement, see, e.g., Fortnightly Corp. v. United Artists Television, Inc., 392 U.S. 390 (1968); Sony Corp. v. Universal City Studios, 464 U.S. 417 (1984).}
See also the Wikipedia article "Copyright Clause".