Actuator availability? Is it true that there is no way to purchase any sort of Technic/PF/PU linear actuator directly from Lego at this time? The current giant Liebherr excavator seems to include #40918, a part which was only introduced last year, but they can't be bought individually on the store, Pick-a-Brick or the Education site. Is this true, or am I missing something? 

I know that they are available on eBay and other secondary market sites, but it seems odd that they cannot be bought directly.
Short answer: Yes.

Long answer:

Technically linear actuators are not part of PF/PU system, so they are treated as generic part. S@H don't sell individual parts apart from PF/PU elements. Pick-a-Brick (PAB) has very limited part selection and linear actuators are never available there as well. I also haven't seen LEGO Education selling any actuators, only PF/PU elements. This brings us to the only possible place - Bricks'n'Pieces (B'n'P). Here you can usually buy most of individual parts from recently produced sets. However some parts are never listed for sale. These are either absent or have a note suggesting that piece cannot be bought this way (for example, PF/PU items). I remember linear actuators being in this grey area so I cannot tell if actuator you are interested in (even though I'm interested in new Technic elements I didn't pay much attention to this piece) was available for sale at any point. Missing/broken parts option, which is similar to B'n'P, on LEGO Service webpage actually has Large Actuator on the list and can be requested if your is broken or missing, obviously. So it may be possible that this item could be purchased on B'n'P after all. However this cannot be confirmed by the time of this answer as Bricks'n'Pieces is unavailable due to COVID-19. 

If you really need the piece - you already found it on secondary market. There is nothing really against using it as prices are often lower than LEGO B'n'P offers. Shipping is also quicker than LEGO could offer.