Your superficial test tested all three of the special cases in the PRIVATE_OPENERS array, which is what the parser.ts code actually checks. DEC's question mark, which is special cased yet further off on its own, is in reality another "private opener", too, and it isn't limited to DEC (e.g. XQTMODKEYS), and neither does DEC not use the other non-digit parameter characters (e.g. DECDA3).
(There's a hypothesis that DEC's own state machine didn't care where these marker characters were, as it was a simple state machine that had to fit in ROM and probably just set a bitflag. A mistake that we're probably all still making is assuming that they only take effect when in the very first position.)
STRING_OPENERS is another widespread special casing that people do, treating ESC plus a few characters as special rather than handling all of the 7-bit aliases for the C1 characters as the general case.
You seem to think that people who share what the mistakes are and where they themselves have made these very mistakes over the years, to help other people not make them and so that the world continues to remember this hard-learned stuff, is somehow worthy of ad hominems, straw men, insults, and vilification right off the bat. That's a very poor show and you should be ashamed.