This is a common phenomenon in the pronunciation of Indo-Aryan speakers. The English alveolar /t͇/ sound is actually alveolar, i.e. it is pronounced with the tip of your tongue at the so-called alveolar ridge, that bumpy thing between your upper teeth and the edge of you hard palate. The interdental /θ/, commonly spelt as ‘th’ in English, is pronounced with the tip of your tongue between your teeth.
The Indo-Aryan languages do not have these sounds, instead they have the dental /t/ with the tip of your tongue at your upper teeth and the retroflex /ʈ/, commonly written in the romanization of Indo-Aryan languages as ‘ṭ’ and pronounced with the tip of your tongue slightly curved back and put roughly between the alveolar ridge and the hard palate.
These four sounds, the alveolar /t͇/, interdental /θ/, dental /t/, and retroflex /ʈ/ are four completely different sounds. So, when Indo-Aryan speakers learn English, they frequently try to find the closest phonological approximations to the sounds they use in their native languages and start resorting to them in their English pronunciation. This is why they commonly use the retroflex /ʈ/ sounds instead of the English alveolar /t͇/ and the dental /t/ sounds instead of the English interdental /θ/, which results in pronunciations like ‘ṭoot’ (‘tooth’), ‘healt’ (‘health’), etc. Keeping that in mind, it is actually easy to imitate an Indo-Aryan accent It is also important to note, that there is nothing wrong with such phonological approximations, e.g. people speaking Slavic languages do them all the time too, using their rolling r’s and dental t’s and whatnot. Heck, I do it all the time when I speak English!
However, in the Sinhala tradition people are mostly unaware of these phonological distinctions, so they really believe that the English ‘th’ sound is equivalent to the Sinhala and Pali dental ‘t’, hence erroneous spellings like ‘meththa’. The truth is they are not, and the story of that Sri-Lankan group believing that people got the meaning of ‘atta’ (‘aththa in their spelling’) and half of other doctrinal terms all wrong because of the spelling shows us that this Romanization had better be avoided to prevent confusion in both the Pali and Sinhala context