I don't know but I guess that the rate for grin is a fixed one, at
0.00675676 BTC via grin, send 2.252253333 grin
At the rate, theymos accepted higher rate than real-time price of Grin on exchanges at the moment.
No one knows where Grin will go next months.
Let's see.
Anyway, it is interesting to see that the forum eventually accpeted another payment gater, rather than only in bitcoin.