This should be clear enough for anyone to understand. Accepting it is what's hard.
Linked for brevity
Clear in the sense that I think I understand what you think. And clear in the sense that I understand that you don't understand globe earth theory. Do a little poking around on the subjects of geodesy and NAVD88.
Thanks for the tip. Take a look at the meaning of level, sea level and see how topographical maps are made and ask yourself why doesn't Earth's landmass have any curvature.
Level: Many definitions, but in 'sea level' it is probably being used as either a noun (a position on a real or imaginary scale of amount, quantity, extent, or quality) or an adjective (at the same height as someone or something else)
Sea level: the level of the sea's surface, used in reckoning the height of geographical features such as hills and as a barometric standard
Topographical maps are made in relation to sea level. On a globe, sea level could alternatively be described as (roughly) a specified distance from the center of the globe. You're attacking a strawman. Sea level on the globe Earth is not a plane, it is a sphere.
On a globe, the blue line indicates sea level. (Rough diagram, I know it's part of an oval, but work with me here.) The two black lines indicate the shores of Australia. The orange line is the 191 mile 'bulge' between Australia's shores. The green line is the highest point on Australia, which is the brown line. Obviously this isn't to scale, but intended to assist. Topographical height is determined by sea level. Sea level is (roughly) a sphere of radius r around the core of the Earth. This is the location height is measured against. Not against the chord/plane represented by the grey line. That would be fairly useless.
You're basing your point on a strawman. Perhaps a misunderstanding on your part, but a strawman nonetheless.