Liz started to dip in polls after she proposed a plan to seriously pay for M4A. Her embrace of M4A tanked her, not the opposite.
Liz has a better chance of passing stuff because she is wants to drop the filibuster, Bernie doesn't. So really it has nothing to do with how they talk, or even how well they grasp the policy issue.
The healthcare plan that will pass if Dems have the majority will look more like Biden's, no matter who is president. This whole start way left and you will get more from the deal doesn't really work in practice. What the last vote in will tolerate, that is what you get. If M4A advocates what to get there, they should start answering the serious question surrounding it, build public support, and advocate for plans that can help us back into single payer.