QDM
The default option is most definitely that a strip pilot presumes the condition of the surface is unknown, whatever the owner may or may not say
Knowing nothing about strip flying I can only guess that this is the case for some and not for others, otherwise why should anybody ever claim for damage?
Flying is a serious business, and a pilot going to land on a runway is likely to assume that it is a runway, not any old field. Otherwise, why not land in the field next door? I guess that somebody has assumed exactly that, and broke something, and claimed.
There is also a thin line between what you perhaps call a "condition unknown" strip, and what somebody might call a grass airfield. Where would you draw the line? Forget what I fly; that's irrelevant.