The airport grounds and the Airport Express right-of-way have not been sealed off. The sort of folks who are doing the sort of things being done in Mongkok are perfectly capable of figuring out how to bring trouble back to Lantau if they ever decide to go that route again. Let's not talk details. We should accept that risk remains, including risk of cancelled flights for transit passengers, so long as there's trouble in Hong Kong.
It's equally possible for something like this to happen at SFO, and yet we don't see a "San Francisco survival guide" talking about the potential risk for transit disruption due to airport unrest. It's about equally as likely.
This guide is supposed to be objective unbiased information, not fear mongering — that's the media's job.
The risk of disruption is not commensurate with the strong wording of the warning.