It is of course an empirical question how well mutual expectation performs (and we shall examine this below) but mutual expectation is not in any sense a significance test.
if we are examining a phrase like the east end, the conditional probability of east given [__ end] or of end given [__ east] may be relatively low (since other words can appear in that context) and yet the phrase might still be very lexicalized if the association of both words with this context were significantly stronger than their association for other phrases.