Correct. And account abstraction already is implemented in the current Casper PoCs.
Edit: sorry, misread what you meant. Multisignatures for security purposes are totally possible because a user can specify whatever signature scheme they want. Multisignatures for signature aggregation purposes are not really feasible because (i) they conflict with the goal of signature abstraction, and (ii) there’s little point because it still takes O(N) effort to process rewards for validators.
Though what we can do to make life easier for light nodes is have them randomly sample a set of voters, and only ask for and verify votes from them. Another thing you can do is use BLS signatures internally inside a stake pool.