Celestial Seasons FAQ [1] says they sell 1.6 billion cups of tea per year, so this would work out to about 5 grams per serving, which seems around the right ballpark for the entire tea bag. Maybe their thinking is that without the staples, the tea bags are compostable, so they don't go in a landfill?
Not all brands, obviously, but don't assume the 'bag' itself is biodegradable.
Anecdotally, certain fancy tea bags survived an 18 month composting cycle, which I had to then pluck out by hand before spreading said compost on my vegie patch.
That number looks about right for "because of this, every one of our teabags is recycled instead of thrown out", which is obviously not true. But it could also be a supply-chain stat, where the landfill mass of those strings and staples is measured from cotton and iron, instead of the finished products.
3.5 million pounds? Of tiny strings and staples? Wouldn't that require literally billions of teabags to reach 3.5 million pounds saved?