This doesn’t necessarily mean the US has less PHDs available

It actually does. That's how math works. If you have X - 10,000, your result is less than X

The movement of highly educated people between countries is a measurable quantity. It is the HFBDI the "Human Flight and Brain Drain Indicator".

Here is one possible source for that.

The Human Flight and Brain Drain Indicator considers the economic impact of human displacement (for economic or political reasons) and the consequences this may have on a country’s development. On the one hand, this may involve the voluntary emigration of the middle class – particularly economically productive segments of the population, such as entrepreneurs, or skilled workers such as physicians – due to economic deterioration in their home country and the hope of better opportunities farther afield. On the other hand, it may involve the forced displacement of professionals or intellectuals who are fleeing their country due to actual or feared persecution or repression, and specifically the economic impact that displacement may wreak on an economy through the loss of productive, skilled professional labor.

Here are some numbers from that (lower is better):

Year: HFBDI
2015: 1.5
2016: 1.4
2017: 1.7
2018: 2.2
2019: 1.9
2021: 1.8
2022: 1.8
2023: 1.7
2024: 1.4

I was not able to find numbers newer than 2024.

Some things to keep in mind:

1.) The US is reliably one of the countries with the lowest brain drain indicator, it is always in the top 10 countries that people move to. This means, even when the US numbers get worse, it is still a brain-drainer, not a brain-drainee.
2.) This indicator is looking at the educated middle-class and upwards. It can not tell us about only top-scientists and researchers, but it can provide an indicator on trends.
3.) Emigration from a country at peace, in contrast to fleeing a warzone, happens on long timelines. People start planning and preparing often years before they actually make the move.

What does that actually tell us?

So the spike we see from 2017 to 2023 is most likely a delayed reaction to something happening in 2015 or 2016.

Trump has a definite effect on migration, fewer educated people move to, more from the US then usually. Still, a net-gain over all in the segment from educated middle-class upwards, but less positive than before.

Now what can we say about top researchers and scientists?

This article provides a look into the minds of Ph.D. students and sesearchers, especially in STEM. Here we find:

Three quarters of U.S. researchers who responded to a Nature poll conducted last March were thinking about moving abroad. The trend was especially apparent among early-career scientists: of the 690 postdocs and 340 Ph.D. students who responded, 803 said they were considering sailing for other shores.

This is not about if they will take on other industry jobs, or go to silicon valley, this is a poll about moving abroad.

Thanks to archive.org, we can look at the quoted Nature article directly.

Many respondents were looking to move to countries where they already had collaborators, friends, family or familiarity with the language. “Anywhere that supports science,” wrote one respondent. Some who had moved to the United States for work planned to return to their country of origin.

But many more scientists had not planned on relocating, until Trump began gutting funding and firing researchers. “This is my home — I really love my country,” says a graduate student at a top US university who works in plant genomics and agriculture. “But a lot of my mentors have been telling me to get out, right now.”

I believe we'll not only see a very much worse HFBDI for the US when the numbers for 2026 and 2027 come out, but we'll see a real brain drain of top researchers, not necessarily the old guard, but the people they trained up to replace them are at least strongly considering leaving and are welcomed with open arms in other countries.