It is true that previously young students in some regions have received instruction in their national language in the first grades of school, and then in Mandarin when they have learned it well enoguh to receive instruction in Mandarin. Now, they will get instruction in mandarin from the start. This is probably a reflection of how proficiency in Mandarin is becoming more common, even among the youngest students. They can however still have classes which teach the national language also, which has also been common previously. The law mentions that this right to the national language should be protected, although it is not very specific about how local party branches should do this, essentially leaving it up to local cadre, often from minority nationalities themselves, to work it out with the people they represent.
It is of course possible to critique this, but it is not a case of the party banning anyone from speaking or learning their national language as such.
Sure, it is a question how to best safeguard the national languages as well, and it is possible they may make mistakes here. Often policy details are left to the regional governments, so hopefully they can find good solutions. My point is just that it is a policy question, and not a “they are commiting cultural genocide”. This is a continuation of the marxist approach to the national question, which often helped build up the national cultures and languages and writing systems in the first place in the time after the revolution.