In The Witch of Blackbird Pond, why does Hannah tell Kit that she "has never escaped at all" by marring William?

Let us be very careful with what the text actually says.
This reference comes in Chapter Sixteen when it looks as if Kit and William Ashby are to
be married. When Kit is with Hannah Tupper, she says that one of her reasons for
marrying William is to escape from her uncle's house and all the drudgery and daily work
that she has had to do since her arrival. However, in response, Hannah
says:

"But
remember, thee has never escaped at all if love is not
there."

Thus, this is a
slightly different situation from what your question suggests. Your question seems to
suggest that Kit marries William in the novel, whereas this never happens. Hannah is
actually telling Kit to be very wary of committing herself to a marriage if she does not
love the other person, because, although escape is what Kit wants, she will not find it
in a loveless marriage.