This is a major misconception, because
merchants don't have to hold Bitcoins to accept them.Why? Because services exists this times that handle the payment/exchange process from Bitcoin to the fiat currency of the merchant's preference and availability inside the service itself right away. Extremely easy.
Example:
https://bitpay.com/retailIs this what I think it is?
You can accept payment and then quickly convert it into fiat right away with the current market price?
No, it is not what you think it is
The poster you replied to seems to mean that merchants don't even see any cryptocurrency per se as they are using payment processors like BitPay or CoinGate, which deal with all the required conversions and volatility issues. Basically, you, as an online shop owner, just install a plugin on your site which is provided by the payment processor, and your clients will be able to pay with crypto. Then at the end of the week you receive a fiat transfer into your bank account which totals payments made during that week by the clients of your shop