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You're forced to use shorter even with the modern, unless you accept IIII for 4. But the other said no 3 consecutive symbols.
The test that includes "yadf" also passes a comparison function.
The 'standard' modern form is, to my understanding, essentially a decimal system so it typically treats each digit of the number individually, which means 1990 is treated as 1000 + 900 + 90 + 0(ignored) as the author has indicated.
Further abbreviations are possible, sure, but I've seen no source that would indicate that they're widely accepted (unless you count Microsoft Excel's fairly unique willingness to display all sorts of weird variants). Do you have a source which suggests that Roman numerals correctly use the shortest possible variant? This would surprise me, given that the original numeric system was even more verbose than our modern one, with subtractive notation being infrequently used, leading to long strings of additive symbols to represent large numbers.