It was meant to be hyperbole. What does assimilation look like? Speaking English? Being Christian? You guys can’t even agree on a single denomination. Eating American food? Corn dogs and deep fried Oreos for all, no pizza, French fries, hamburgers, and sorry Chicago, no more polish sausage.
And since you’re all not Puritans, it’s safe to say that assimilation hasn’t been the name of the game since the 16th century.
America has always been a melting pot since the beginning
Yes and no.
Our best immigrants started their American experience with a healthy amount of shame and a desire to abandon their old ways. Every ethnicity other than the original Dutch and English colonials in America were embarrassed and wanted to conform. Many refused to let their children speak their ancestral tongue in the home, most gave their children "American" names or even shortened their last names to assimilate. Metaphorically, they burned their ships so there was no going back to their native lands, they were going to be Americans or die trying.
The reward? America respected them and their American accomplishments. Decades later, they weren't the dirty lowlifes they were considered when they arrived in America as the underclass nationality of their era. Everyone is Irish on St. Paddy's Day. Everyone enjoys Cinco De Mayo. In Chicago we all eat the Polish cake with a plastic baby Jesus inside. In Wisconsin virtually 100% of people go to fish frys on Fridays even if they don't observe religious Lent.
But what about the multiculturalism of today? New immigrants keep their language, they name their children their native names, they move to neighborhoods intentionally to hang out with "their own people". This is not a melting pot strategy, this is an invasion and colonization. They also do not adopt American morality and standards, they retain their former ways which inhibits trade, employment, and trust. Not good.
Examples of stereotypically excellent immigrant communities in America? Persians and Cubans. They hate what their countries have become, they love what America is, they assimilate all the positives of this country and they abandon all the negatives of their homelands. Bonus points to one of my favorite people, the Koreans. Judeo-Christian values and excellent neighbors, industrious, and they love America. I've met several Koreans named "Henry" because their parents wanted their children to succeed in business just like Henry Ford. It says a lot about what they value as a people and how beneficial they are to the American way.
Not so great immigration? Soviet Bloc immigration has had mixed results. Some flourished in the American way, others were experts at gaming their old system in order to survive and are experts at gaming our system with so few checks and balances. Latin American immigration has also been problematic because the worst dregs of their societies got into our nation with no verification. They've taken over entire cities and have brought lawlessness and cartels with them. Case and point: there are more El Salvadorans in the USA than are in El Salvador. MS13 members are amongst them.