Two thoughts:

1) The amount of flex in the rear triangle is minuscule compared to the amount of give in the tyre, and in the case of the Brompton, the rear suspension block. The material of which the rear triangle is made, is immaterial. The fork has more give to it, but it is still small compared to the tyre contribution. So for first line comfort, the tyres dominate completely.

2) The material properties can have an impact on how impacts are transmitted through the frame to the rider; a springy material will have a more pronounced filtering effect compared to a stiffer material. So high frequency portions of road impulses will travel less well through titanium than steel, and this may have the effect of feeling less jarring. It is the high frequencies that cause discomfort. For the short tubes of the Brompton, the effect is probably too small to be detected by the seat of your pants.