Yes they are, but I'm glad you figured it out! I've left my Hoyas outside for the winter here in central Florida and they all did pretty good except for imperialis which has now resumed growth after quite a bit of dieback. The lowest temps we had were two back to back nights of 37. Usually we are in the 40s and 50s at night. Warm winter nights can be in the 60s. This was the warmest winter in years.

Hoyas that were minimally affected by the cold: compacta, bintulu Borneo, calycina, Serpens, subcalva, hat som paen, kanyakumariana, shepherdii, cagayanensis, epc 233, australis, skinneriana, odorata, cagayanensis

Hoyas that do not like the cold at all: imperialis, coriacea, cv joy (all have resumed growth)

Macgillivrayi had minimal dieback, resumed growth quickly. Buotii drops leaves and lacunosa gets ugly and yellows but both pull through just fine.

Some are a lot more cold hardy than others! You can only find this out the hard way ;)

Someone needs to tell my plants it's almost Winter, they are still putting out new growth!