The remarks come in response to a passage in Professor Lawrence H. White’s
book, “Better Money: Gold, Fiat, or Bitcoin?,” which argued that Bitcoin’s fixed supply makes it too volatile to serve as a medium of exchange.
But Saifedean Ammous, the author of the 2018 academic book “The Bitcoin Standard,” said that Bitcoin already functions as a medium of exchange and that volatility’s effect on that is likely to diminish.
So that was the remark of Saifedean Ammous in response to bitcoin volatility because of it fixed supply whatever.
I've not read what they both said in details, but with the little you've shared, I could deduce some realities in both of them, even though they might not explain them to understanding of most people.
From Professor Lawrence H. White, I believe he meant the plausibilty of an aggravated demand-induced volatility that would be primarily caused by the inherent fixed supply of Bitcoin. By medium of exchange, he might not necessarily mean the lack of exchange, but the elaborate one where most people would focus on Bitcoin. If happened, the volatility would be chaotic.
From Saifedean Ammous, his own view is not a direct link to Professor Lawrence H's remark. I believe he is staying the present reality of Bitcoin, and also hinted on what could happen in the future if it continues this way. The "diminishing" speculation is not new, as it has been speculated by different important personalities, and some have been discussed on the forum.