What is the effective equivalent battery capacity between Lithium Ion and Sealed Lead Acid batteries?
There is a germ of truth in this, though the details will be specific to particular batteries.

Li-Ion batteries maintain their capacity fairly well for several hundred charge/discharge cycles, discharging to most of their capacity, provided they are not overcharged, or left discharged for any length of time, or discharged below a certain point. Tesla claim over a thousand cycles I believe, but to what Deplth of Discharge (DOD) I don't know, and they have invested a lot studying cell chemistry and charging.

(There are other batteries capable of several thousand charge/discharge cycles - LiFePo4 sometimes used in heavy vehicles, or Lithium Titanate (LTO) with tens of thousands of charge cycles, at which point storing mains power becomes economical)

But even "deep cycle" or leisure Lead-Acid batteries don't last very long if you discharge them below 50% of their capacity. (An occasional deeper discharge followed by immediate recharge is probably OK). But if you want hundreds of cycles without much loss of capacity, 50% DOD is pushing it. Call it 40% and that's pretty much in line with your salesperson.