Even 1960s audience's could clearly see they were wide open at the bottom.
I already addressed that when I compared these suits to the spacesuits seen in "Tholian Web." If the script said cold weather gear, then the costuming department shouldn't have produced "salt shakers" that would not be recognized as such by a 1960s audience.
Granted, the suits were not sealed shut and taped against biological contamination, but they do look like some kind of isolation gear, like radiation suits. Considering the contamination seen right in the teaser, it is natural for a viewer to think the exotic attire was meant as iso-suits. If the costumers had gone with heavy coats (like those seen in THE WRATH OF KHAN), then the audience wouldn't have surmised that Tormolen was a pinhead.
There is also Spock's line, "Be certain we expose ourselves to nothing."
I didn't read every post, but at any rate at the point where he reaches his hand up under the hood to his mouth to warm it with his breath--I knew it wasn't an 'isolation suit' even when I was a kid.
You are right they could have done a better job at making clear that they were suits against the cold and not biohazard suits, but the script is clear so the intent made sense but the execution was off.