Is it really a lie? Paraphrasing, it's like 'Let me take care of her because she's too dangerous for you.' Nowhere do you say 'I will kill her because like you I think all Githyanki are evil,' but that meaning could easily be inferred.
Should that line even be tagged [Deception]??
Yes, it should - It is Deception.
Deception ≠ Lying. Deception encompasses lying, but it's a much broader concept, and the sort of person who wheedles to say "Oh, but I didn't technically 'lie', so I'm in the clear!" is, in most cases, a proliferate deceiver.
In this case, the player is, without question and with no shadow of any doubt, deceiving the tieflings. It's a deliberate deception, and it's right to be rolled as such. You can rationalise and justify it in your head, but that in itself is dishonest - you know what you're doing, you know what you're implying and allowing them to think with your choice of words in this particular situation - the fact that you didn't 'technically' lie is utterly irrelevant to the fact that you are deceiving and need to roll deception. Asmodeus does the extreme vast majority of his work with truth, and without ever telling any lies at all; in almost every case, every word he speaks is true, and he deceives through that.
If we want to look at some other divine patrons, try Tymora - she's as good as it's possible to come, more or less, but she loves a good trick, sports the trickery domain, and is openly aware and appreciative of the means by which trickery and deception can be used to do the right thing when it matters - as arguably we do here, to diffuse the situation and split up the fearful-of-difference tieflings and the bloodthirsty non-gith-are-lesser-beings gith.
