Belichick is far greater than Lombardi. It's not even close.
Lombardi won 2 Superbowls.
His other 3 championships were before the SB era and compare more to a conference championship than Superbowls because the league had half the number of teams.
Even if we assume they would have won the SB 3 more times Bill still has 6 and 3 other conference titles.
I hear what you are saying, but will slightly disagree.
Lombardi and the Packers were still competing against every other coach, and every other team. He and GB were still better than all the other clubs. It's not as if there was no other good coaches or good teams; Paul Brown and the Cleveland Browns come to mind right away, for example.
However, there IS indeed a mile-wide difference when comparing the two coaches and two eras. Lombardi (and Brown, Landry, Shula and Halas)
never had to deal with the salary cap or any genuine free agency. During their time it was very easy to keep a good team together for a long time, which resulted in their dynasties (and legacies). It also resulted in bad teams rarely moving up to threaten the elite, and if they did so it took a very long time to accomplish that goal.
The double-edged sword of free agency (1993) and the salary cap (1994) was supposed to make it easier for bad teams to move up and topple the king of the hill. Everybody would get to take a turn being the champion. No more long periods of being bad would generate more fan interest (and therefore more revenue). Belichick obliterated that narrative.
Belichick was way ahead of his peers in terms of making what was supposed to be an obstacle into a useful tool for building and maintaining a roster. Shula for example retired right after (1995) those rules were put into place - probably not a coincidence. Lombardi once traded a player literally five minutes after he found out that the player hired an agent to negotiate his contract. Would he have been able to adapt to free agency? Perhaps he would adapt, but I'm guessing he more likely have taken a low-cost approach, and would have been more on par with the Hugh Culverhouse Tampa Bay Bucs of the 80s, or the Bidwell family Cardinals.
Those guys listed above are all good coaches, and still would have been innovative and found ways to come up with winning schemes relative to their peers. But I also believe their winning numbers would have been significantly diminished if they had to play by the same rules that Belichick did.
To only look at the raw numbers without considering the significance of the cap/FA when comparing Belichick vs Lombardi et al is disingenuous. It's no different than if one were to compare old-time QBs like Johnny Unitas to modern QBs. If one only looks at the numbers without considering context, then Unitas is the 25th best QB all time, Joe Montana is #24 - and Kerry Collins is #23.