I thought that flask was supposed to work the the 45mm Nalgene. I know that it says, "Please note that this flask will not fit the 500 ml Nalgene bottle top filter unit we carry", but it looks like all of their Nalgene bottle-top only units have 33mm necks.
They have 45mm neck Nalgene bottle top units, but they come with the flask ... and the flask has the same pressure warning.
Guess I'll just go with 33mm unless I find a decent 45mm collector for a good price.