I believe this question is based on a misunderstanding of the poorly-expressed claim (which seems to be plagiarised on many sites).
Compared with the distortions caused by First Past the Post voting, Preferential voting gives an advantage to minority parties:
Dr Peter Chen from the University of Sydney said preferential voting allows for a greater number and diversity of candidates.
"In the Australian system using the numbering preference system it can mean that a less popular candidate can be eliminated and the general preferences that the elector has can be redistributed until one candidate receives 50 per cent plus one for the lower house of the votes," he said.
"And Australia really introduced that with an attempt to, in a sense, facilitate a more diverse range of political parties and candidates to run and it's associated with in a sense the growth of some of the minor parties in Australia."
However, the Weebly site listed in the question isn't comparing First Past the Post and Preferential voting.
It is comparing Proportional representation (in the form of a Single Transferable Vote) used in the Australian Senate to elect all of the Senators, with the Preferential Voting used in the Australian House of Representatives to elect individual representatives.
By example, if in every electorate, 40% of each vote for (i.e. give their first preference to) one major party, 40% vote for a second major party, and 20% vote for a minority party:
under preferential voting used in the House of Reps, each candidate will effectively be chosen by the minority party voters' second choice. They will all be one of the two major parties.
under proportional voting used in the Senate, the number of each parties' candidates will approximate the 40%:40%:20% split.
While the Preferential Voting system avoids much of the "tactical voting" associated with First Part The Post, the system is not flawless. Indeed, Arrow's impossibility theorem suggests it must necessarily have deficiencies.
