70% of 100M is 70M. There are not 70m 18-30yr olds in the USA.
50% users under 30 (gen z). only counts 18-30, estimate as you will for those under 18 (to be fair she only referenced gen z): https://explodingtopics.com/blog/tiktok-demographics
total users in US is 170M: https://backlinko.com/tiktok-users
your call on if 85M users 18-30 is "[gen z] society".