That would depend on how you define an illusion wouldn't it?
From the perceptions of those that looked at whoever was encased in the ice and when they (the spectator) touched the ice to see if it was real, the ice would be 32 degrees F/ Zero degrees C. That is cold. With that perception of touching the ice and it being cold to them, the illusion of it HAS to be cold inside would be imprinted on their mind.
Could have DB been snuck out and in with a false bottom? A 'trick'? Perhaps, but why? The only reason I could think of would be so he could get rid of body fluids (you know #1 and #2), but even then, if he were to go without any intake of solid foods and little amounts of hydrating fluids, would having to rid the body of waste be an issue? It was a publicity stunt for our entertainment and IMHO think that he would have stayed inside, using the perceptions of the cold illusion.
It is not going to do any good to debate the subject for we all have our own ideas and perceptions on it, none of which are wrong for each individual. I can accept your input and understand that it is your belief and I respect that. I ony offer my perception and belief from my own experiences and thoughts.
Just so mine are out there:
I don't consider it an "illusion" if he did what he said he would do. "I will remain encased in a large block of ice for several days." If he wasn't in the ice - trick, if the ice wasn't really ice - trick, if he left periodically - trick.
Personally, I think that if there being a loophole (heat generated inside the iced in area could keep him warm) doesn't change the fact that he did what he said he would do.
Anywho, as you said it depends on the ay each person defines "illusion" and "trick".