You are correct. The workforce engages because it is something that they see is beneficial to them. They must be accountable to becoming engaged. So on that point I agree wholeheartedly.
However, I must disagree that walking the talk compromises engagement. Your point about not trying to force someone down a path is a good one. But, in the beginning, it is important to for folks to see that their leaders are doing what is being asked of them. Although I am not as experienced as some, I have been doing this work for a number of years. In my experience I have never seen an organization sustain a high level of engagement that has definitive impact on its productivity and profitability without the senior leaders taking action on engagement. Now you are correct to say, that may mean they must stop doing something that is disengaging the workforce. But, just as the workforce must choose to be engaged, they must choose to not be disengaged. It is important for the senior team to be engaged because that sustains those high level benefits to the organization. At some point, the employees will not care what the senior team is doing. But, the senior team should. They are a team too.
The goals of senior management are going to be very broad. It is up to the front line managers to translate these goals into something meaningful for their lower level employees. Therefore any buy in from senior management is going to have a little eventual effect on the employees.
If senior management is very unfocused then frontline managers can provide focus by giving specific goals to individual employees, and therefore build engagement. And if senior management has very good ideas about employee engagement, front line managers can undermine this by ignoring their employees, or translating senior management intentions badly.
Therefore front line managers are the weak link in any engagement initiative.