"Open onto" vs. "open into"

These Google ngrams, for

• open onto the living room
• open into the living room
• open onto the garden
• open into the garden

support the sensible explanation you put forward.

• Open onto the garden seems to be used more often than open into the garden (I'm sure this would be more pronounced with 'patio', most easily pictured as a 'flat space'), whereas
• open into the living room seems to be used more often than open onto the living room (I'm surprised people use the latter).

Of course, there are 'the doors are open into the garden' and 'the doors open into the garden' tokens commingled in these results.