It is not because of a long running DML and small rollback segment.
Well, I think they meant "long running query" instead of "long running DML". Little misconcept.

In a few words, SNAPSHOT TOO OLD = Long running queries + small rollback segments (or little undo space) + DML on the data being queried. Mainly SELECT + DML together, the problem may be minimized but not completely solved with bigger segments or more room on undo tablespace (unless one have infinite disk resources, however this workaround is pretty stupid).

The problem isn't the SELECT or the DML, but them together.