Cecil shrugs. What's the deal here? "Well, a library's a library. It was the day before yesterday, that I arrived," he thinks back on it, and oh right, he was meant to be keeping a secret, oops. Good on Ianto for reminding him, and he adopts a similarly hushed tone. "But I have to stop and correct myself here. The TARDIS doesn't exist. Ships that travel in both space and time aren't real. Though apparently time travel has been at least partially de-criminalized, and space travel, as I'm sure you're aware, is predominantly a side effect of corporeal existence and so unavoidable for most of us anyway." Ianto seems familiar with the TARDIS, so it's probably okay that he's aware of her existence. But better safe than sorry.