I've just used ASE 16.0 SP02 PL02's Interactive SQL to import a tab delimited data file with no problems. I created the data file by BCPing out of sysindexes. I created an empty user table version of sysindexes, by doing "select * into b from sysindexes where 1 = 2". All of the rows were imported in to b with no problems.
I also used it to copy data from one table to another. That is a useful part of the tool. Using BCP to import data from a table in a different ASE server would either need a data file to be created to hold the data exported from the source table, or the use of a pipe to BCP out and in simultaneously.
I cannot say why the import tool was added to Interactive SQL, Maybe to make it more tempting to users of paid for GUIs?
I would reiterate the point that the reply posts in the old link that you supplied made : namely to use BCP to import data from files. If you can use Interactive SQL then you should be able to use BCP in a command window. Granted, it's not a GUI interface but it is more configurable than the import tool. For example, there does not appear to be a way to change the row terminator in the import tool, so it probably won't be able to handle data that spans multiple lines for a single row. BCP can handle data like that because both the column and row terminators can be specified, using -t and -r respectively.