Recruit isn't the same as making Thomas her Knight. No, she isn't lying about trying to recruit him, but she can recruit all she wants knowing full well it cannot happen.. Is she lying?
I'm not sure what that even means. Recruit means make him her Knight, or at least that she would definitely offer him Knighthood. She might (and I say
might) be able to 'offer' it to someone she knows cannot accept it, but she would have to present her 'offer' supercarefully to get around the 'no falsehood' compulsion, and Thomas (or whoever) could easily force her to tell the truth with even a simple direct question. It's an extreme stretch at best.
I'm pretty sure Mab
can't get around the compulsion by 'qualifying' a statement later. That would reduce the compulsion to nothing, it would be trivially easy to get around and nobody would even bother referring to the idea that the Sidhe can't speak a falsehood.
She can carefully phrase direct factual statements to
imply a lie, or leave important information out, or otherwise lie with the truth, but the statements themselves must be factual at the time, or at least she has to truly believe they are factual at the time.
There just isn't much wiggle room in Mab's recruitment statement.
There is also several quotes in Cold Days over three pages which explains more or less as to why she may threaten to recruit Thomas, Mab isn't serious about it, even if it were possible, it's Harry that she wants. This is where Mother Summer opens Harry's eyes to what Mab's real job is, and what is on the line, makes it easier to understand why Thomas just isn't suitable for the job.
page 331 Cold Days;
Mother Summer is now hitting home on what Mab really does to Harry..page 332
Finally on the next page Harry gets it.. 333 Cold Days;
This is why, though maybe she wasn't telling a lie about recruiting Thomas, she was still bluffing. Thomas isn't a starborn, Harry is.. With the BAT coming Mab needs a Knight that can deal with Outsiders, that is Harry, not Thomas..
Yes, she wants Harry. I'm sure she sees Thomas second best on several levels. But if Harry is dead (really dead, I mean, gone and not coming back unless God wills it), then Thomas would be her next choice, because then she can't have Harry anyway. Or at least, Thomas would have been her choice at that time, whether that's still true years later is another matter.