You're just assuming a flawed implementation of a predictor and complain that it's flawed. That doesn't invalidate the general premise that predictors are compressors.

Predictors don't have to give you some corpus-average probability. They can give probabilities adjusted per symbol. The probability could even be exactly 1 for some symbol when the predictor is absolutely certain (and then it costs 0 bits to encode).

If you had a magic oracle predictor that already knew all the data you wanted to send, you'd have perfect compression.