Irregardless of the fact that the complaint this quote comes from is from somebody who's party trick it is to complain about there not being enough race cars in every update of the game, the bolded simply doesn't exist. If there was a 'balancing concern', PD wouldn't have tried to shove a square peg in a round hole when they added Group C cars to Gr.1 in amidst semi-modern LMP prototypes and concept cars. They wouldn't have shoved in a 90's GT1 car in Gr.3 when the rest of the class is semi-modern GT3 cars, or facsimiles of such. And the biggest one, they wouldn't have shoved in what is a one make racer into Gr.4, instantly making said one make racer the bane of the entire class for months until it was BOP'd to oblivion.
From what I have seen? 'Balancing concerns' don't exist in PD's mind. They seem to just shove cars into whatever category best fits what they currently have (and from this update I can already tell the 300 SEL will be dumped into Gr.X oblivion and more or less ignored like every other classic racer they've added in) and whether due to the inability, or lack of desire, to completely restructure how the game handles it's car classes, it reflects really poorly on them. So yeah, I really doubt there's such a thing as 'balancing concerns' with regards to adding new racing cars to anything other then Gr.X, they simply just don't want to go through the hassle of invariably restructuring their broken class system, on either the road or race car side.
Look, idk what exactly it is that's keeping more Gr.4 and Gr.3 cars from being added, but I think there's a connection, here. Gr.4 and Gr.3 have the least additions compared to Gr.1, which has a ton, and the former two classes also have had the most BoP changes whereas the other classes don't get updated nearly as often beyond when new cars are added to those classes. There's also the factor of how Gr.4 and Gr.3 are the most often-featured class, especially between both FIAGTCs.
I'm really just trying to guess based on the limited information we can get, here. And I think they put cars into classes based on the typical times they set compared to the current cars in those classes. We also don't know how much is being done with GTS that hasn't been planned out somehow already by PD, versus issues that could very be addressed in the next installment. For example, I think the Vulcan ought to be in Gr.X alongside the Zonda R and P1 GTR. (Indeed, I'd personally make a class just for these track day-only hypercars.)
But I think PD is somehow unable to "transfer" cars to another class, either due to limits of technology, or manpower. (As I'd bet most of PD is working on the next title, and its subsequent add-ons.) I think PD should also make at least a couple classes for vintage racecars - perhaps one for historic prototypes and another for historic grand tourers, respectively such as the Ferrari 330 P4 and the Shelby Daytona Coupe. At any rate, I believe there's a limit on what PD is able to do regarding classes in GTS, and especially the creation of new classes and transferring cars to different classes.
On a similar note, I wonder if we'll eventually get interiors for the cars that don't have them (namely some GT6-era VGTs like the Mercedes and Aston Martin VGTs), or if that could be something reserved for the next title.
I'd also like to add that I think the current class system is fine, and is more self-explanatory than the PP system. That said, I think it just needs some adjustments, rather than scrapping it. For example, I think the S-FR Racing Concept wouldn't be as much of a menace if we could add aero parts and rear wings to cars, and it seems like the N-Series effectively uses cars with added parts, anyway, like full-customize parts when tuning is enabled, or being able to change BB/TCS/FM when many cars aren't able to change that on the go when left as-stock.
As for now, I think it's better to have more limited selections of cars for the N-Series races, much like how we saw some Daily Races with the "Gentleman's Agreement" cars. Perhaps another N-Series event could have us using the sporty pickup trucks that are in-game, or an N200 race that's limited to the various hatchbacks. I am getting a bit tired of the N-Series races just being either one-makes, or allowing all cars and thus leading to just a couple cars dominating. But that said, in the context of GTS as it is right now, I'd love more one-make N-Series races with stuff like the S-FR Concept; you wouldn't need to worry about it dominating other cars if it's the only car that's eligible, right? Heck, a one-make Sambabus race could be fun, too. I'd legit love to design some Daily Races/FIAGTC rounds, tbh.