So how does Texas have standing then?
Voters do not select the President, states do, by nominating electors in proportion to their representation in congress. The Constitution specifically calls for each state legislature to make the laws which select those electors.
If a majority of those electors choose a person, then that person is the President.
The problem is if a state does not follow the US Constitution and chooses electors not by laws passed by that state legislature. Note there is nothing in the Constitution which says the voters of the state vote for those electors. The fact that citizens vote for which electors to nominate is a newer phenomena, as states legislatures used to be the only method used to select electors.
All other states are then placed into a situation that means their own state and its electors are disenfranchised from a Constitutional determination of who is selected as President should the electors of those states who brought electors to vote used extra-Constitutional methods.
Texas and the 18 other states submitting briefs all have been disenfranchised for who should be selected as President so they most certainly have standing in the only court in the country which can hear its case.
Make an easy example here: If Georgia's governor decided on his own that he would select all electors to vote in the electoral college, then that would most certainly be evidence of disenfranchisement.
