From the homepage, Pricing → Commissions → Forex leads to the 1,000,000,000 cutoff.
To clarify the terms in that table: one basis point is 1/100 of 1%, as they themselves clarify in a footnote rather than assuming all readers know it. And trade value is, well, the amount you trade. :) The footnote for that term clarifies that they convert and charge this commission in whatever currency you have configured as the base currency for your account. (For example, I've chosen USD for my base currency since most of my transactions are US stocks on US exchanges, even though I'm in Canada and therefore the legal entity I'm officially dealing with is IB's Canadian arm.)
So, to convert anywhere up to US$100,000 (or its foreign currency equivalent) to any other currency, they charge their minimum fee of US$2 - mathematically, 2 / (1/100 of 0.20) = 100,000. Above that and up to US$1 billion, you take the amount you're trading, multiply by 1/100 of 0.20, and that's the fee.
$2 is a ridiculously low fee to convert five USD figures or less of currency, given that they also directly pass through the very tight exchange rates collectively quoted by their suppliers without building any profit for them into the rate itself.
But yes, they can exchange even a dollar or two. I've exchanged five figures a few times there between CAD and USD, as an American living in Canada and actively using a US credit card.
So far I've found them to be much more affordable than TransferWise, but equally, they are a very complicated broker to use, and it may not be worth the hassle just for FX. They have other benefits that make them worth the tradeoff for me, like good margin interest rates and the ability to provide tax forms complying with both the US and Canadian tax systems each year.