Texas sued because other states blatantly violated the Constitution of the United States with their internal business as you put it. NJ or any other communist fascists states cannot sue Texas as Texas gun laws follow the Constitution. They could sue Texas to require Texas to set forth LESS restrictions as it relates to 2A (Constitutional carry etc.).
SCOTUS decision to not even allow for arguments to be presented is disturbing and sets a very dangerous precedent for all upcoming elections. This will go down in history as perhaps one of the biggest SCOTUS blunders of our history.
It appears that lawlessness prevailed. You can cheat, burn, steal, urinate on the Constitution and much more it appears. The walls have been breached.