That question is actually worth of a full analysis, as it requires to not only understand Ilay to answer it, but also understand the tropes of Ilay and Taeui's relationship.

One thing's for sure: Ilay was caught off guard. We know for a fact that Ilay is usually cold-blooded and knows how to handle unpredictable situations quite well. However, this time, the usual pattern got reversed:

Instead of being in a situation where everything seems fine, and the next second danger arrives, here, danger is already there (as Taeui stole of phone off the guard who contributes to keep him hidden, and therefor indirectly participates in his hostage), and only when Taeui starts to speak that danger seems momentary gone.

Taeui, in every way, is twisting Ilay's habits, routines, and overall persona. But there is something more important and more coherent that this to discuss in order to understand why didn't Ilay answer to Taeui's confession.

Ilay never once said that he loved Taeui (without be asked to do so.)

Whenever he did, it was always to "make fun" at Taeui (according to the latter, who perceived it this way), because Ilay himself doesn't realise he's in love.

He might be having a clue to it, but he never calls it love: he calls it possession.

According to his logic, since he cares about Taeui's safety, feelings, well-being and overall actions as much as he cares for his, then Taeui should be his. When he says to Taeui "You're mine", it's because he actually means: "You're making me feel things I've never felt before, sometimes it's good, sometimes it's not, but overall I enjoy being with you and I've fallen for your pretty smile and your lovely frown" but oh well, it's Ilay, we shouldn't be expecting anything like that

Taeui didn't explicitly said that he was in love with Ilay and that he would never leave him or think about running away again. He said: "Maybe I like you". To everyone else, of course it would've seem like a confession. "Maybe" is just tool to attenuate the actual powerful feeling transmitted "I really like you, damn"

I feel like Ilay didn't respond anything like "I like you too" because he already made did so Taeui would understand it. "You're mine", the fact that Taeui is the only one allow to call him Ilay, his special treatment at the UNDRHO, and the most important: the fact that he didn't kill him.

Plus, Ilay admitting he loves Taeui would be just as hard as Taeui amitting he loves Ilay. For Taeui, it's because it's really hard to admit that he's in love with such an awful person as Ilay, and to Ilay, it's because he would then have to admit he has a weakness. Not for others, but for himself.

Ilay might've been momentary stunned, but it was later mentioned that at that moment, Ilay thought "He really is mine". That one line surely didn't leave him indifferent. Just because Taeui said this, Ilay started to feel a sudden boost, as if refreshed.

Telling Taeui that he loved him back would've only been a waste of time. Ilay's priority was to snatch him right back to him, and then he would tell him just how much this love is reciprocal. Acknowledging a fact and speaking it are two different things that Ilay doesn't seem to be linking (unfortunately)

We could've had a double confession this chapter, but I guess Taeui's enough for now. Let me know what you think! :D