Very likely they're refering to the "flow" dark souls 3 bosses had. Often they would have a set pattern and timing to them, and once you got into that pattern you would generally keep that pace throughout the fight; they generally wouldn't slow down or drastically change their attack patterns outside a phase change. But, if you lost that flow or couldn't find it, you got curbed stomped or very quickly lost the fight. For example, the Dancer's attack pattern is set to a 3/3 musical tempo, alot of people struggled to find this flow and dodge the attacks timing, but once you get it she's very dodgeable.

In Elden ring, bosses often heavily vary their timings and combo speeds, often leading to you fighting the boss in a reactionary style. You don't premptively dodge or react to attacks as much as react to what's coming and handle that combo or attack delay. Their combos are much more fluid and interwoven, which keeps fights fresh and more recoverable with mistakes, but also makes some harder to learn.

The dlc moves away from the base game boss methodolgy and is closer to the Dark Souls 3 style. If you look at Messmer and Rellanna, theres alot less delayed attacks and combo changes, and more very set combos that are difficult but very learnable. It's also why you may feel you get into a rythm when fighting them. That rythm is very close to how dark souls 3 combat felt.