No, it doesn’t mean that. The churches are not in communion, which is not the same as there being excommunicated.
In spite of what you read, the Roman Catholic, if he “joined” an Eastern Orthodox church likely wasn’t excommunicated so much as he went into schism. He could just go to confession, I believe, and resume attending a Catholic church.
Generally, the Catholic church requires that a Catholic satisfy his Sunday and Holy Day obligation at a Catholic Church. If he or she is in a location where there’s no Catholic Church he may, but doesn’t have to, attend another church with valid sacraments. Catholic churches will give communion to the Orthodox, but they prefer that the Orthodox, and members of other churches that have valid sacraments but which are not in communion with Rome, go to their own churches if they can, as they do not wish to create trouble for such people. If they approach to receive communion, however, they will be given it.
This contrasts with Orthodox churches which vary in their positions. Some churches will give communion to a Catholic if the Catholic approaches, some will not.
Conversely, various churches treat their members who do that differently. As noted, Catholics are required to go to a Catholic church on Sunday if they can. However, if they were somewhere on Sunday and there was no Catholic church, and they went to an Orthodox church, they don’t have to say anything to anyone about it. For some Orthodox churches this is also all true. In some places in the troubled region of the globe the amount of intercommunion is very high. However, some Orthodox Churches (perhaps Oriental Orthodox, I can’t recall) will regard a member who receives communion at a Catholic church as placing themselves outside of communion in their Orthodox church and no longer freely receive them for communion.
Interesting things, I’ll note, happen with baptism. Catholics regard any baptism done in the proper form with the proper intent as valid. At least some Orthodox, however, will require a Catholic entering the Orthodox as requiring a new baptism.
Note: Not a canon lawyer so all this is subject to correction.