Maybe it's not so much about being completely invisible as being hard to localize by thermal imaging?
For instance, if they can dump most of the heat into the air, so that what the thermal imager sees is a big blob, then if you had several tanks advancing as a loose group, so their thermal blobs overlapped, someone trying to use thermal imaging to aim an anti-tank weapon might not be able to tell where to aim to hit the tanks instead of the spaces between the tanks.
For instance, if they can dump most of the heat into the air, so that what the thermal imager sees is a big blob, then if you had several tanks advancing as a loose group, so their thermal blobs overlapped, someone trying to use thermal imaging to aim an anti-tank weapon might not be able to tell where to aim to hit the tanks instead of the spaces between the tanks.