Marriane's notions of cold-heartedness and Austen's assessment of John Dashwood are surely two very different things.

As Katherine Reeve points out in her introduction to the recent Bath Bicentenary Edition of S&S, Marriane is typical of the eighteenth-century cult of 'sensibility', which promoted a notion of superiority of feelings and aesthetic understanding. Marriane would perhaps view anyone who didn't share her passionate feelings as cold-hearted, what she fails to realise is that Elinor is trying to spare both herself and her mother by refusing to give them false hope. Elinor thus plays down her feelings-perhaps this is something she has had to do all too often.
However it's also possible to add that by managing their disappointment she is also sparing herself.