Canon has a long track record in doing this. Back in 1985,
they built all the T90 ever sold within only three months in
a single production run and then sold it over a five year timespan.
They subsequently perfected this, and only once miscalculated,
when they ran out of stock for the D60 before the 10D was
ready to ship.
The 5D Mk4 started shipping after 2.400.00 copies preproduced.
Looking at the fact that many R5 across the globe are now
in news embargoed hands of ambassadors, journalists and
influencers, they are for sure running production at maximum warp.
And they will have enough parts sourced for that in advance.
After all, they are the only camera maker who can afford this
without risking to implode.