In my experience of hiring engineers across several companies over the past decade, the pool of senior SDEs who are both not already in the US and willing to move to the US is so small that I'd be surprised if it even registered in the overall H1B flow. Virtually all the non-resident senior SDEs I saw getting hired in my orgs ended up being H1B transfers who got hired straight out of college into the US and grew into seniority while here.