Zell applies explicitly between mechwarriors, and does not include combat vehicles or PBIs. Battletech understandably focuses on combat between mechs, and this shapes our perception of the Clans, but that doesn't mean that the Clans behave the same way toward planetary milita. In fact, we are told they do not.
I'm also having trouble imagining "never a step back" on a tactical level. Take Unzmarkt, The Clan is attacking trying to break into or out of a particular beachhead. They are facing an armored division! So does the Star Captain say something like: "Well strategically, we are trying to drive back the enemy and vanquish the division, but tactically, I am only trying to eliminate just one more company. Those other companies that are flooding in to fill the gap in the line, they are not part of my tactical objective. So my work here is done, and good thing too, because (not coincidentally) my auto-cannon is empty and my armor is low (otherwise I would decide to take on another lance or two, tactically that is)." The IS forces are described as having a "strong defensive line" that "held back" the Clan for three days. So the Clan was trying to break the line, but couldn't have done it in one continuous action. If they attack and fail to break through the line, how do they break off the attack with enemies to their front without "tactically" retreating? But never mind retreating, it is nearly impossible to imagine that they were constantly advancing, yet couldn't break through for three days.
I think it makes more sense to see the constant advance as the Clan's preferred tactics. In many cases it was successful, but when that wasn't feasible or failed, they would either call in reinforcements (and presumably wait for their arrival) or stop advancing. Furthermore, I don't think the Clans saw this as particularly problematic after the first few waves of Revival. The honorable tactics of Clan warriors were meant to be used against their equals, not virtually unlimited numbers of tanks and infantry backed by artillery. Zell does not even apply to them, and the Clans soon reasons why the IS should not be treated the same as Clan opponents.
Even the Smoke Jaguars did not always charge headlong into battle.
Albiero: "Backed up against the ridge line the militia's armored battalion used artillery to trap the Clan forces." Invading Clans Sourcebook, p. 66 (again from Sarna). So the SJ's were unwilling to continue their attack up the ridge, in fact, it sounds like they retreated to cover for a time.
Port Arthur: "They (SJ's) immediately engaged the First Proserpina Hussars at Zuave Vale, forcing them to fall back to secondary positions at Disher. Here the Smoke Jaguar attack bogged down, forcing saKhan Weaver to order a "headhunter" Star to locate and destroy the First Hussars' command company." Invading Clans (sourcebook), p.69. How did the attack become "bogged down" if the Clan is always advancing until one side is defeated? Furthermore the SJ's were "forced" to change tactics, suggesting they couldn't take the secondary positions with an immediate direct assault even if they wanted to (which you know they did).
During the reprieve in '50-'51, the IS had time to prepare against the Clans with intelligence and training from Wolf's Dragoons, yet the Clans were still generally successful against the Successor States because they adapted to the IS's style of warfare. Besides, I think the OP's question is more interesting if we assume the Clan has some tactical flexibility.