Agreed and the those thoughts aren't mutually exclusive. The over-rated HS recruit doesn't know that he's over-rated. Probably 90% of them feel that they are under-rated.
But there are also Joe Burrow, Justin Fields, Jalen Hurts, Kyler Murray talents that were sitting on the benches at these schools until they decided to go somewhere where the depth chart said that they had a real shot to play. I don't think any NFL scout would say those kids were over-rated HS recruits. They just didn't do their homework in choosing a school with a depth chart that meant that they weren't going to get the opportunity that they wanted. The elite talents sitting behind other elite talents at QB is a bit more pronounced because there is only 1 QB that typically gets the majority of the snaps but it is happening at other positions as well.
And you are correct, not all of the top 4 and 5 star kids that transfer will pan out with a less crowded depth chart at a new school like the examples that I provided. But a reasonable % will and just didn't do their homework when selecting a school out of HS.
For example, Ohio St has like 8 top 100 overall WRs committed in the 2020 and 2021 classes including both classes #1 WR and the #2 overall 5 star WR in the 2022 class already. Some of those kids are extremely talented and not over-rated but will never be the man at Ohio St. One is already in the portal after one season but he won't be the only one. Meanwhile, Penn St had 2 true freshmen WRs rated significantly below every all 9 of those Ohio St WRs that I mentioned that started as true freshmen with Washington making some freshmen All-American teams and lists of most impactful freshmen. Washington did his homework, was probably under-rated, and went to a big time program with a depth chart that was screaming for immediate playing time. The 8 top 100 WRs I mentioned committed to Ohio St barely saw the field and only did in one game when like 20 some kids were out with COVID. They did not do their homework.