This is just my own speculation, read with caution 
I did some research on previous forza game releases and realized that Forza Motosport 2023 uses the same engine as FM6 and FH3, maybe with slight modifications. [horizon games uses same engines as motosport counterparts, but with x1.625 global acceleration multiplier, that increases car acceleration and gravity acceleration = x1.625 more grip]
All u need to do i too look at how forza aero parts fine tunning works.
In FM6/FH3 like in new motosport forza aero always adds the same values on top of car stock downforce, so most of old cars always got 34/62 [KG] with forza aero on default settings, and if car generates, and if car stock aero generates any downforce, it will be added to forza aero default values, this does not include lift/downforce from ground effect.
In FM7/FH4 forza aero always showed the same values regardless if stock car had any downforce or not [exception were cars with stock adjustable aero] and it always was 75/100 on default settings [i donât remember what units tbh], i think that most of cars generated lift/downforce from âground effectâ and it affected whole car but not specific axle like in previous game, that would mean that cars were more oversteery compared to fm6/fm 2023
In FH5 forza aero MIN values were always equal to stock car downforce x1.1 [that also includes âground effectâ downforce, for old cars cars overall downforce was always the same, the only thing that was changing was overall lift car generated that was not included in forza aero values.
Thats probably why we are not getting SOME of fh5 cars in fm 2023, becouse cars are not compatible backwards 
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How many cars in FH5 are in FM23?
The question is how many we donât have, including GT race cars. U can always adjust cars to fit into this game⌠but thats more work than it would if fm didnât use outdated forza engine.
But your original post said we arenât getting FM5 cars in FM2023⌠at all.
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Itâs been a year and everyone knows the game launch was very disappointing. This title was roasted for a whole year and for a good reason. Still lots of things that need to be fixed⌠BUT I just donât see how making stuff up and manipulating helps anyone.
I myself am a bit salty that FH5 gets 992 GT3RS before FM but in this situation it just feels like you are looking for a reason to complain (especially having read your other comments in other threads). Last few updates were pretty good so I say just let them cook.
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Im not complaining tho⌠[at least not here] i just stated what i found out lol.
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I hope T10 has learned Not! to Release a Game that isnât ready & do way more testing on tyre & handling model it feels to simplified it needs to be toned down a bit & innovate not just on development but but more closely with the most vocal in the community the executives should stick to there own Role & let the development team do there Role to many executives interfere with things they donât Understand in Development XD
The engine will undoubtedly have a global physics system which varies from game to game. Car models will be modelled individually. Each game has a different physics âfeelâ with that in mind the numbers displayed are arbitrary and only related to that particular game. That would be my take on it anyway. The numbers in the tuning set up are often way out anyway, in all of the games. Weight, power and torque seem to be the only numbers that are relevant. Lat Gâs, brake distances, accel speed and aero are rarely on the money.
For all of FMâs missteps I do think this is the best weâve had in terms feel. Perhaps a bit understeery on stock
Cars but is easily tuned out. Oversteer is predictable and manageable and lift off oversteer, which plagued mid engine cars in 6&7 in now much,much better imo.
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Youâre just speculatingâŚ. Maybe or maybe not.