I doubt that the consulting firm seriously overbilled.

To my knowledge rather consulting firms are great at selling the necessity of lots of consultants or consultant days:

Just let the customer talk very openly about their wishes for the project, and you immediately get an insane scope explosion for the project, i.e. it "needs" an insane amount of consultants over many years to implement all these wishes.

To increase the bill, every highly qualified consultant that is necessary for the project "needs" a lot of support personnel (senior consultants) so that the senior consultant can 100 % concentrate on their work (otherwise the customer would pay insane hourly rates for highly qualified experts to do "grunt work" - no customer would "want" that). This way, you sell a huge number of senior consultants (this is rather some low rank) to the customer.

And, by the way: since of cause many consultants you sell to the customer shall be highly qualified experts in their discipline, and the project trivially consists of a lot of disciplines, the number of subject-matter experts that can be sold to the customer can be increased by a lot. In some ordinary software project, you would simply use a small team of good generalists (jacks of all trades, master of none) who can do most things in the project, but of cause, as a consulting company, you rather sell the customer "some of the greatest experts that money can buy" (without mentioning that these are insanely expensive and not really needed for the project).

That's how you do it; scamming or billing unrealistic hours is for amateurs.