I don't care where the investment came from, a VC, your dad, or someone being their own VC. If we start calling that bootstrapping, we need a new name for 'starting a profitable business with nothing but a 5 bucks VPS'.
It is not '1. Become a VC, 2. Invest in your own company.'
That is literally what they did with their first company...
This is no different to launching multiple products under the same company.
These guys ran their first company for 1 1/2 years, then went full-time and seemingly focused most of their energy on this new product.
This is basically a pivot or new product. There are lot's of ways to frame this approach which are analogous to 'properly' bootstrapping.