It's a bit more murky and nuanced than that, though.

What is executive power? Well, it's what the constitution says it is. But a lot of things in the executive branch aren't spelled out in the constitution. A more strict interpretation of it (one that SCOTUS seems to be moving toward, at least when it's convenient to achieve conservative policy goals) is that Congress should not be delegating so much rulemaking power to the executive branch.

So let's say you're the SEC. You're an executive branch agency, but you essentially write laws. You get to write those laws because Congress delegated that power in some reasonably specific ways unique to your agency. So who do you really report to? Nominally, day to day, seems like the president. But you only exist because of an act of Congress. Congress can dissolve you tomorrow if it wants to. Congress can also give you more power and more latitude (within some limits, of course). So while you report to the president, you're accountable to Congress. That might mean that the president isn't allowed to fire your people without cause, if Congress specified so.

That's not an interference of executive power if the power you wield actually flows from the legislative branch.