This isn’t quite true. The person moves towards the impact in a crash. Racing drivers are likely to have a frontal impact, ie crash into the car(s) ahead and thus their head move forwards whilst body is held in position by their harness. Of course, the impact forces will be less than if travelling in opposite directions ie head-on crash, but at racing speeds those impacts are likely to be comparable to a head-on road crash.
Seatbelts have very sophisticated timing systems. Once a crash is detected, they will slightly expand and contract in a specific sequence to draw the person into the seat without whipping them back or crushing their chest.
Harnesses attached to car seats don’t have these timing systems (which are integrated with other car systems like airbags and crumple zones so they all work in unison).
However, they do spread the impact forces over greater areas of the body, which is generally safer, especially on smaller bodies. They are also better at holding the body in position than seatbelts. Internal decapitation is a risk with extreme head movement but it’s also a risk of an incorrectly fitting seatbelt (ie not on shoulder correctly) even at relatively low speeds.
I presume (but haven’t checked) that racing car harnesses are integrated with the car systems in a similar way, so have the advantage of timing systems and spreading the forces.
So it’s not as simple as ‘harness = bad, seatbelt = better’ - there are lots of factors regarding size, weight, positioning that will all come into play.
Rear facing is safer in the most dangerous crashes - frontal impact where vehicles are travelling in opposite directions (most impact energy) as they will hold the body and head against the car seat. However, for the vast majority of driving, head-on collisions are either lower speed (around town) or unlikely at high speed (motorways / dual carriageways have segregated traffic. If most of the driving someone does is on unrestricted A roads, ERF is probably very sensible. If it’s mostly 20 roads around town, the risk of forward facing with a harness is less (but erf is still better).
Parents should be informed enough to make their own risk analysis and buy the best / safest seat their budget allows. I just had a quick look and couldn’t find an ERF seat for 4+ for much less than £400.