No. You weren't. Or at the very least you were arguing against 'mechs in general. It's not like an Artillery 'Mech magically requires supplies while an ARC-2R doesn't.
No, from the outset that's what spurned this question, if the terrain blockage is big enough that artillery can't shoot over it, how are you getting supplies to the 'Mechs on the other side?
Armed Buildings and Infantry Field Guns say you're wrong.
I'm surprised you didn't mention trailer as well. Armed buildings aren't really mobile, and I don't know if infantry field guns count as towed guns.
You know what's better than taking hits on your armor? Not getting hit at all. You're trying to strawman this into a discussion about toughness when I'm arguing mobility.
Well you (or someone else) seemed to suggest that. And if your using off-map artillery mobility doesn't matter at all, can't move the unit.
A really heavily armored artillery tank is going to have to compromise on speed and probably armament, too. And it depends on having optimized choices of positioning and setting up before the fight.
You don't need much, 4/6 should be enough. Artillery in BT (and a lesser extent RL) is a bit weird that way, you only want to move far enough to not be hit by splash from your previous location, interestingly this assumes that they hit your last location.
And remember that TMM doesn't apply to artillery, you don't need to move far between each shot, just enough to not get hit!
In the end, mechs go places & do things that tanks can't.
Like walk across the bottom of a river, or out of a dropship on an airless moon, etc etc.
Having a single Catapult in the Command Lance of a Battalion might be just the tiny edge they need to open up a hole in defenses that are otherwise restricted to standard range guns.
Just to make sure you understand we're talking Arrow 4, Thumper, Sniper and Long Tom here, not LRMs.
So we're talking about a river at least 5 kilometers wide, such that tanks with LT's on the far bank can't really help support your advance. How are you getting supplies to these 'Mechs? If those tanks can't get across, how are supply trucks going to get across?