They have no argument

No because the feather will be affected by resistance to the air. However, if feathers are bundled to weigh the same as the cup, they will fall at the same time. Weight depends on density, buoyancy and air resistance.

In a vacuum, free from resistance, both objects fall at same rate. Amazing. Ask Galileo, he got some things right

The concept of the constant acceleration for different objects due to gravity (at the same height and ignoring atmospheric effects) is usually attributed to Galileo. In reality, Galileo merely popularized the concept with his Tower of Pisa Thought Experiment (What happens if you drop a cannon ball and a musket ball from the top of the Tower of Pisa?).

Quote from: cassini on September 14, 2017, 01:35:48 PM

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Now answer this, jellyfish.  If I drop a cup and a feather, both land at the same time. Where does Mass/Density come in here?

NOT in the air. DUH.

That only happens in a vacuum.

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The vacuum is a clarifying tool, removing ambient air to remove the influence of friction due to air resistance.
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This means that without the air removed it is impossible to tell by simple observation which falls faster, the feathers or the bowling ball.
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If we want to know whether ambient air has any effect on our experiment we need only compare results between this test done in a vacuum and this test done with normal air pressure present.
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If you knew enough to post "Not in the air; duh!", then you knew what cassini meant.

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Please don't give flat-earthers credit where none is due. HAHAHAHA
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