Man that statement makes me feel old, but Sega is the biggest, their growth happened in the previous century, decades ago.
Sega genesis, which I'm randomly assuming was the best selling product of this group of companies, got to about 30 million players. Nowhere near 700 million revenue. To put it mildly, this was not something they could sustain.
So I have little doubt that Roblox is indeed worth more than the sum of these companies. I don't like the games either, sort of a second life but for kindergarten, but clearly they are commanding more success in the market than those companies.
Ubisoft alone makes more than $2 billion in revenue per year.
edit: Square-Enix is another $2.3 billion in revenue. Capcom is around $700 million per year. And Sega is even more at $3.4 billion per year.
And presumably the people who just invested half a billion at $30G expect it to go significantly up from that after the company goes public.