Not true IMO since gravity acts as a field. The moment the field “arrives” at the planet it creates a force that is directed from the planet’s center of mass back towards the sun. As long as the sun stays put during the eight minute transit time everything works out.
“As long as the sun stays put during the eight minute transit time everything works out.”
Therein lies the problem, the sun does not stay put but is acted upon by the mass of the planet, which causes it to ‘wobble’. That is, the sun and planet both orbit around a point which lies on a line joining their centroids, not at the sun’s centroid as would be the case if the sun staid put. Sure, the point is much closer to the sun than the planet due to the vast difference in their masses, but it’s not insignificant either (trying to avoid getting into the maths of it here). It is this angular change which would cause the gravity vector to shift off the perpendicular at the planet’s orbit and cause it to start losing angular momentum if gravity didn’t act instantly.