Yes I got the suggestion, but that is being too literal. The features of Sanskrit that make it “Perfect” or “Perfected” grammatically were already there in the early and middle Vedic stages, and even before the Vedic stage i.e. in the Proto-Indo-Iranian and Proto-Indo-European stages of the language. Most of the grammatical and phonetic features of Sanskrit were inherited from past stages of the linguistic tradition, so the language was gradually getting morphologically and phonetically simpler but not by much. It was still comparatively very conservative compared to other Indo-European languages, in preserving its inherited IE features.
Pali exhibits many of those same features in its underlying grammar and phonology as are present in Sanskrit, but it has some simplifications and innovations (vis-a-vis Sanskrit) in its phonology and morphology that are not gradual/evolutionary.
The sanskrit grammarians do not view Late Vedic (Classical Sanskrit) as a different language from early-Vedic (Vedic Sanskrit) even though they recognize that Early Vedic is an earlier set of dialects not currently spoken in their time, but they could still understand middle Vedic which was spoken about 4 centuries before their time (or roughly 4-5 centuries before the Buddha’s time, as Classical Sanskrit was being natively widely spoken the Buddha’s time).
After the vedic era there was large scale linguistic mixing from the Iranic and Dravidian border lands causing not gradual/slow evolutionary natural evolution of the language but an abrupt linguistic car-crash leading to the emergence of Middle Indic as a distinct linguistic stage. This runs parallel to the political changes leading to the collapse of the Janapadas (regional independent kingdoms) and the emergence of large Empires like the Achaemenids, Indo-Greeks, Mauryans, Sungas, Kanvas, Kushans, Satavahanas, Guptas etc transformed the political and social situation entirely in the 500 years after the Buddha’s time.
Most of the new aspects of middle Indic (which are not present in Old-Indic i.e. Sanskrit/Vedic) are not evolutionary natural changes but the result of the Iranic mixing with Indic and the further changes brought in by the introduction of writing the language non-phonetically using scripts not originally suited for 100% phonetic representation. However, Classical Sanskrit and its literature continued to exist and thrive (in a diglossic situation beside Middle-Indic) all through the middle-Indic stage until middle-Indic itself disappeared leaving behind early forms of the modern Indo-Aryan languages which are spoken in present-day South-Asia. By the start of the modern Indo-Aryan languages (circa 1000-1200 years before now), Sanskrit had become an educated-man’s dialect (which had to be consciously learned as a second language), unlike in the Buddha’s time and earlier when it was the native language of all Indo-Aryans.
So as the speaker in the first video above says, the word Mleccha (which means “those who dont speak clearly or meaningfully”) were recognized as foreigners (speakers of non-Indo-Aryan languages) which Sramanas like Buddhists Jains and Ajivikas were usually not - they were predominantly Indo-Aryans (although their Indo-Aryan speech had evidently been influenced by those foreign languages to a greater degree than grammatically correct ‘Sanskrit’ - whose ‘Sanskrit identity’ is merely its more conservative retention of its inherited structural IE morphological and phonetic properties).
This is why (in my understanding) several Early Buddhist sects did not think too much about restoring their Canons to Sanskrit after a period of having them in Middle-Indic - and the Pali canon has sakaya niruttiya put into the mouth of the Buddha to prevent Theravadins from demanding the same Sanskritizations of their canon as that of the other EBT sects (as the Theravadins wanted to preserve the texts in the middle-Indic form which, after some attempts at phonetic and morphological standardization, became Pali).