I would like to ask the following questions and would be happy to hear your opinions:
1) Is it legal to "assume" a certain runway condition as dry or damp if you are sure - even if the reported RWY condition is WET?
Let me ask you another question:
If the ATIS reports the runway dry (aka
not being wet), and the rain starts to pour down. Do you use dry or wet figures for your calculations?
To return to
your question. The ATIS is an observation at a specific time (in the past). Why would you get the idea (which document) that it legally binds you to using a specific choice of conditions for take off? It's just a source of information to take into account.
What legally matters is the
actual condition of the runway:
Looking at EU-OPS 1.530 (CAT.POL.A.205 - or what ever they call it today) you are obligated to take account of:
"the runway surface condition and the type of runway surface;" (sic)
Nowhere does it mention which source should be used to get that information.