Yes, I found that strange too - they're liquid propellant engines, even those on expendable rockets actually have a service life that would suffice for several launches, in general. That should be good enough for multiple WZ-8 missions, perhaps with some refurbishment in between. There are two possible explanations that I'm entertaining:
1) The engine is not a bespoke-developed type but an existing design which was adopted from an obsolete, mass-produced rocket and there is an ample stock of stored engines in a warehouse somewhere. In this scenario it might simply be cheaper to chuck them in the bin after each flight instead of performing refurbishment, especially if the latter would incur a lot of effort. Toxic propellants such as N2O4/UDMH could complicate matters, for example, meaning the work might have to be done in a controlled building and wearing protective gear.
2) There isn't actually "the engine"
What if this thing has expendable (possibly solid propellant) boosters to take it up to higher speed/altitude after launch from the H-6 and the engines we see are merely sustainers (and reusable)? Perhaps the reference to single-use was with the boosters in mind, but because there were none in the parade config the statement is mistakenly thought to apply to what we see? Consider:
Heat shields against booster exhaust? (h/t
)
Attachment points for the right-hand booster?