Strike says its free to send money via Lightning - how is that possible?

[Bitcoin]

Strike says its free to send money via Lightning - how is that possible?

I was watching Strike's announcement that their API can transfer money by converting from fiat money to BTC, transferring that over the Lightning Network and converting back to fiat at the merchant. The video is here https://youtu.be/o73fWsqJDdY?t=304

But given the spread when buying and selling BTC, how is this possible? Won't the consumer have to spend more to buy the BTC that will then be converted back to fiat at the merchant?

Source: Strike says its free to send money via Lightning - how is that possible?