It seems unlikely the episodes were shown at any screening, secret or otherwise, prior to the Restoration Team receiving them in early June. In the words of Steve Roberts: "There were old tape joins in all the episodes, which came apart in our hands and which could not have survived being put onto a projector or telecine for viewing." So the possibilities are:
(a) Roberts is fibbing for no evident reason (and given how big reports and denials of secret screenings are in current rumor discussion, you'd think someone else who was at the con would have used the actors' statements in a "Gotcha!" moment);
(b) Watling and Hines misremembered when the screening occurred (but again, someone else would still have brought it up);
(c) what Watling and Hines said at the con has been misunderstood.