Musk only owns 40% of SpaceX so he's not the majority shareholder.
The judge's order says otherwise (https://noticias-stf-wp-prd.s3.sa-east-1.amazonaws.com/wp-co... page 35):
"O grupo econômico de fato liderado por ELON MUSK, com atuação em território nacional, engloba entre outras empresas a X BRASIL INTERNET LTDA, a STARLINK BRAZIL SERVIÇOS DE INTERNET e a STARLINK BRAZIL HOLDING LTDA, diretamente ligadas à SPACE EXPLORATION HOLDINGS, LLC, cujo controle acionário é de ELON MUSK, que detém 50,5% de suas ações, sendo que 78,7% das ações com direito a voto lhe pertencem, conforme demonstrado no comunicado da Federals Comunications Comission, obtido no endereço eletrônico [...]"
Though it's possible that the judge and you are looking at different companies; you might be looking at the USA company, while the judge might be looking at a separate holding company which is the parent of the Brazilian companies.
My understanding of the case, from reading the original orders from the judge, is not that their assets are comingled, but rather that both are under direct control of Elon Musk (him being the majority owner of both SpaceX and Twitter), and therefore form a single "economic group". AFAIK, there are some situations in which the law allows going for assets of other companies in the same economic group, and that seems to be what happened here: Twitter did not have enough assets to pay the fines, so assets from the rest of the economic group have been frozen to ensure the fines will be paid.