The 9mm Silvertip of the era, when put through the FBI protocol, failed.
Sure. Those bullets weren't made to handle intermediate barriers like sheet metal, glass, etc.
However, two facts remain.
1. The particular bullet in question, did actually penetrate sufficiently to meet the FBI's minimum penetration specification since there were no intermediate barriers.
2. Trying to pretend that the outcome of something as complicated as the Miami shootout all relates to a single bullet is absolutely ridiculous.
More to the point, it's only one of many thought exercises one could engage in.
What if the bullet had penetrated another inch?
What if the bullet had been FMJ and penetrated another foot?
What if the bullet had been aimed a foot higher and hit Platt in the head?
What if the bullet had been aimed a bit lower and took out Platt's elbow?
What if Dove had scored more than 3 hits before his gun was disabled?
What if Dove had a backup gun?
What if Gordon McNeill who was right on top of Platt and Matix when the cars came to rest had been armed with a high-capacity 9mm instead of a revolver and had been able to neutralize Platt before he even got out of the car instead of expending the 6 rounds in his revolver and then getting shot while trying to reload? As it was, he scored 2 out of 6 hits, incapacitating Matix for the duration of the fight. What could he have done with another 7 or 8?
What if Manauzzi hadn't lost his sidearm during the car stop and had been able to participate in the firefight?
What if one of the agents had been armed with a long gun at the outset and had neutralized Platt before he got out of the car? What if all of them had been armed with long guns?
What if Hanlon hadn't lost his primary revolver during the car stop and had been able to use it instead of only his compact backup revolver?
What if Matix hadn't been neutralized early in the fight and had been as effective as Platt?
What if Platt had been wearing body armor?
And so on...
Even if one chooses to "what if" only based on that single bullet, there are still a variety of ways that it could have changed the outcome of the fight--penetration wasn't the only thing that one could focus on.
The FBI chose to focus ONLY on one bullet and then to focus ONLY on the penetration of that bullet. That's out of the many, many things (definitely including tactics) that could have dramatically changed the outcome of the fight for better or for worse.