It is unclear at best. The ToS do mention that they are '...all for use in conjunction with Nest hardware products' so I'm not trying to be deliberately mislead

It is unclear at best. The ToS do mention that they are '...all for use in conjunction with Nest hardware products' so I'm not trying to be deliberately misleading in claiming that they relate to the operation of the smoke alarm. I'm sure that Nest don't mean anything bad but the ToS and its legal reach is far from clear. This (to my knowledge) is the first safety device that Nest have produced, the ToS had been written earlier to make it plain that you shouldn't rely on their other products for safety. They need to update the ToS for the new product, basically.