Aloha zebra,
The journey to getting into a the lotus position for your meditations can be one of healing all of it's own. Remember that just like being a vegan or vegetarian, the ability to do the full lotus does not a good or healthy person make.
The sitting postures section isn't the "end all" of preparing yourself to work into the full lotus, however it is suitable for most situations. Take your time. Do the stretches (I'd suggest rubbing your hands warm and sending energy as they hold the hips and knees during this), and work your way through the easier sitting positions on your journey. Just as someone with back issues wouldn't be doing the dolphining of the body to the extent and speed Chunyi does in the course, nor would you want to rush yourself into a posture your body has not been prepared for. Working with a reputable yoga instructor in this area is advisable as well.
There are a lot of reasons why Master Lin talks about the benefits of being in the lotus position during meditation. Some of which are mentioned by our friends above. One big reason that hasn't been brought up yet it the opening of the superior bone and sacrum. This area affects the flexibility of the entire body and is key to not only healing of the lower half of the body, but the health of the whole.
If you are expecting the seas to part and a heavenly choir to ring in once you've got into it...well, it's not in most cases going to be like that. Will you be taping into experiences and energy that are perceivable as well as those that are not that you would not have had with a regular sitting position? Yes.
Are you going to get benefit from doing the active exercises and sitting meditations in the best way you can, even without being able to sit full lotus? Yes.
Let me recommend you first start by looking at how, what, and where your are currently sitting for your meditations -and right now in front of your computer while we're at it!
straight back, feet flat on the floor, shoulders relaxed, hips above the knees. You'll be amazed at how sitting in a posture-friendly chair, rather than a "comfortable recliner or couch can improve your meditation, if only because your bottom isn't sinking below your knees and your back isn't slouching. Your Chiropractor has all sorts of information and support s/he can give you regarding your posture and everyday sitting.
Take a minute to stretch and warm up your hips, knees, and ankles as Chunyi shows, before your sitting meditation-even if it's just upright in a chair, and start on your road to a better sitting posture.
Much Love,
Shawn