This phrasing seems odd, considering that those two countries are firmly in East Asia, and would not be considered as a part of SE Asia by any definition.
Well to me they’re part of an overall southeastern quadrant, with the northeastern quadrant occupied by Russia. It’s just how I think about things. If you divide it into eight directions plus a central region then it’s clearly East Asia, which is the common way to look at it.
It’s a habit of thought that I should break because it’s not really a common way to look at Asia, which makes communication difficult. It’s almost certainly a habit derived from discussion about the US, which is often discussed as four quadrants plus “the midwest,” which is really the center.
It might be how you think about things in your own mind but for many other people South East Asia as a region is a pretty commonly understood term with a fairly specific set of countries and basically considered to be the tropical part of Asia where it's always hot and humid. (Vietnam, Laos, Cambodia, Philippines, Malaysia, Indonesia, Singapore, Thailand, Myanmar, Brunei).
This phrasing seems odd, considering that those two countries are firmly in East Asia, and would not be considered as a part of SE Asia by any definition.