Newer startups will probably be BUSL from day one. What we're seeing now are ZIRP startups reaching the limits of their old "open-source it and they will come" strategy.
In a post-ZIRP world, many public tech company valuations have been slashed, and tech IPOs have slowed. Not-yet-profitable private tech companies can struggle to raise funding rounds now at favorable valuations, compared to their last funding round. If you're a VC-funded open source company with no deep moat, this is not a good time. I agree with the GP, we'll likely see more new companies choosing non-open-source but still source-available (e.g. BSL or SSPL) from the start.
I guess they mean startups funded during the past few years when US VCs were awash with cash.