Yes. Martin Luther left the Catholic Church having been excommunicated.
Correct, he was excommunicated. He was kicked out. Thus, he didn't leave..... The RC Pope split his denomination, Luther did not. The RC Pope kicked Luther out, Luther didn't leave.
Of course, this wasn't the first time this happened. Bigger examples happened in 451 and 1054.
Luther started his own denomination in 1521 AD
Not technically correct, but in general yes - these Christians continued to worship God and to do so in communion with others. From that eventually came denominations - so that in Europe there were now Orthodox denominations, the RC Denomination, and the Evangelical ones. Soon there would be the Church of England Denomination are various Reformed Denominations. When Christians congregate together - we have congregations. When congregations associate together - we have denominations.
Who believes Martin Luther's doctrines today?
No one.
Lutherans do not follow Luther, they follow Christ. The words of Luther are not normative for Lutherans, the written words of God are.
Thus it is shown that prior the 1521 AD the Catholic Church already existed.
Yes. As did the various Eastern Orthodox Denominations and the various Oriental Orthodox Denominations. Plus a plethora of congregations not owned and operated by any denomination.
the Catholic Church existed before he did.
True. So what?
You evade the point: It's the RCC that split itself. The Roman Catholic Pope at the time excommunicated Luther and thus split (schism) his own denomination. If the split disturbs you, take it up with that Pope and the denomination he lead.