Agreed. And one more consideration is that (extended) grapheme cluster boundaries vary from one version of Unicode to another, and also allow for "tailoring." For example, should "อำ" be one grapheme cluster or two? It's two on Android but one by Unicode recommendation and is the behavior on mac. So in applications where a query such as length needs to have one definitive answer which cannot change by context, counting (extended) grapheme clusters is the wrong way to go.
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In fact, the name "extended" grapheme cluster should give it away. There was already a major revision to UAX #29 so that the original version is now referred as to "legacy". Your example is exactly this case: the second character, U+0E33 THAI CHARACTER SARA AM, prohibits a cluster boundary now but previously didn't [1].
[1] Relevant specifications: https://unicode.org/reports/tr29/#SpacingMark and https://unicode.org/reports/tr29/#GB9a