re: Can someone elaborate/explain on the Highly Compensated Employee Limitations for a 401k

$135k does seem low for this designation (its now $150k this year). There is an additional point of also being in top 20% at your company.

From IRS:
Highly Compensated Employee - An individual who:

Owned more than 5% of the interest in the business at any time during the year or the preceding year, regardless of how much compensation that person earned or received, or
For the preceding year, received compensation from the business of more than $125,000 (if the preceding year is 2019, 130,000 if the preceding year is 2020 or 2021, $135,000 if the preceding year is 2022), and $150,000 (if the preceding year is 2023) and, if the employer so chooses, was in the top 20% of employees when ranked by compensation.

Subtle brag alert.. I’m over $150k but definitely not designated as a HCE