English is NOT a "constructed" language, because that implies intent.
English is more of a trash heap of things we borrowed from all those other languages that over time people have grafted rules onto to try to decode and standardize. :-P
But (and I say this in the nicest possible way because it's my native language), English is a dog's breakfast of bits and pieces string together with loose rules and exceptions which require you to know from which language we stole the various bits and pieces.
That was his point.
Languages evolve. Every single language has idiosyncrasies like English, dialects, slang and so on so forth. No language is pure.
Words and definitions change over time because their usage and the requirements of them change over time. Fantastic used to literally mean "out of a fantasy" as in it cannot be real, not simply "really, really good" as it does now. If an ordered, constructed language like Esperanto caught on, within a generation or four it too will have irregularities due to colloquial usage, slang, misunderstandings and people who haven't learned the language properly (especially children and infants).
Also, its not just English that adopts words from other languages, you'll find whole English words inserted into other languages.
In fact, it's English's ability to survive being mangled that leads to its popularity. Get an American, Indian and Cockney in the same room and they can still communicate despite using radically different dialects and styles. English is a very fault tolerant language.