Descobri que, para acessar um registro, é preciso ter privilégio de "pesquisa". Eu mudei o ACL para:
dn: olcDatabase={1}mdb,cn=config
olcAccess: {0}to attrs=userPassword by self write by anonymous auth by * none
olcAccess: {1}to attrs=shadowLastChange by self write by self read by * none
olcAccess: {2}to * by self read by * search
Usando isso, consegui consultar (objectClass=posixAccount)
sem mostrar outras contas.