Is "sealing in the flavor" an actual thing? Someone mentioned searing food to "seal in the flavor". My response was that searing was a way to add a delicious crust, and nothing more or less. However, I recently noticed a frozen food mentioning their ground beef product is "flash frozen to seal in flavor".

These are really just buzz words, right? Even if you're talking about juices as opposed to "flavor", you can't seal them in, can you? When the product rests the juice will be redistributed, won't it?
Searing on a grill to "seal in juices" has largely been disproven.  Meat loses juice at roughly the same speed regardless of searing the meat first.  Searing does produce the Maillard reaction and caramelization which enhances flavor; however, searing first doesn't produce better results.  A test performed by Alton Brown in 2008 demonstrated that searing at the end of the cooking process loses less water than searing at the beginning.  For more information see this link.

For the Alton Brown results see this link (video).