A NSL can compel the recipient's silence in the form of a gag order, but the government cannot
compel speech outside of very particular circumstances, such as consumer warning labels, that don't include verifiably false statements.[0] Thus the concept of warrant canaries, though they haven't been challenged in court.[1] There are other articles on both out there, but the EFF links are decent starting points if you're interested in reading further.
Making an explicitly false statement to preserve the silence about an NSL or its subject matter goes well beyond a gag order. The gag order compels your silence; it doesn't compel you to become an active participant in maintaining that silence beyond not acting to break it, let alone persuading people to believe any explanations or stories to explain it or shield the investigation the letter is a part of.
But there's another issue beyond the constitutional problems with trying to order Apple or its officers to make verifiably false statements about the existence of Chinese spy chips: any statement about said spy chips would constitute a material fact to shareholders. And it'd be a a really big one with an effect potentially measured in billions of dollars of market value, multiplied by the fact that Apple lied and destroyed every iota of credibility on privacy issues they've developed over the years. Making that statement--even if ordered to do so by an NSL--would be a blatant breach of the their fiduciary duty to shareholders and SEC regulations.
Even if compelled to do so, Apple (or its officers) literally can't make that false statement. They'd have no choice but to fight an NSL that orders them to make false statements, and even if they lost, they'd limit themselves to "no comment." So while they might be under a gag order on a given subject due to an NSL, if they're commenting about it, we can reasonably accept those comments as true. Or, at minimum, that they are their own words rather than the government's. As a hypothetical, it's plausible that they could be lying about the chips; but if so, it would be because of their own deliberate decision to do. Not that that's very likely, of course.
0. https://www.eff.org/deeplinks/2014/04/warrant-canary-faq
1. https://www.eff.org/issues/national-security-letters