I thought the same thing as you, but it seems that at least some TB3 devices work with TB2 hosts, e.g. the LG 5K display: https://support.apple.com/en-us/HT2074

I thought the same thing as you, but it seems that at least some TB3 devices work with TB2 hosts, e.g. the LG 5K display: https://support.apple.com/en-us/HT207448 https://support.apple.com/en-us/HT207448