Methinks, Joanna, you're going to get various degrees of 'informed opinion' rather than straight fact. Here's mine, FWIW...
It used to be said that ivy kills host trees by strangling them, but then that was made out to be unsubstantiated lore. At the same time it was stated that death occurred by the ivy overpowering its host by weight (especially when wet), perhaps helping bring it down in a storm. But I don't know of any substantiating evidence for that.
AFAIK, ivy is an epiphyte, not a parasite as such, though it obviously competes for nutrients, water and light, and it could harbour harmful pests (or beneficial ones). As such, it may weaken its host through competition.
I suspect most of us have seen tree trunks growing over a constriction or obstruction of one sort or another, eg. fence wires and rails. Yet I have never seen such overgrowth happening where a tree is encircled with ivy - not even where the ivy forms a complex interlocking mesh of branches. That leads me to presume that ivy does not strangle its host, though exactly how the ivy adjusts to the increasing girth of its host's trunk, I can only hypothesize.
I am not entirely surprised by the panic flowering post-severing of the ivy's trunk.
The ivy may have continued to obtain some moisture from its numerous aerial roots attached to its host's trunk. It may even have put out proper (feeding) roots into litter in bark crevices. And the loss of circulation from the roots may have altered the chemical/hormonal balance, resulting in a promotion of flowering activity. That's similar to how, if one has an apple tree that's producing too much vegetative growth, one can stimulate flowering by partially 'ringing' its bark. ...And slightly less similar to how nipping-out the growing tip of a shoot can promote the development of side-shoots (that happens because the tip contains meristem tissue, which produces auxin and strigolactone hormones, which ordinarily suppress the growth of lateral buds).
Let's see what other folks have to say...
Mike