"123456" is numeric, it is not a number.
That is semantics, is not a number numeric then?

The spreadsheet function ISNUMBER is not the identical equivalent of VB's IsNumeric.
Clearly it is not, but in what way should it be, why is it different. Your reasoning for IsNumeric returning true

The string "123456" is a numeral, the string representation of a number. Hence, it is a numeric string.
Q.E.D. IsNumeric("123456") = True
cannot be used if ISNUMBER doesn't do the same. TEXT(1234,"@") is just as much a string/text representation of a number as your VBA example.

I for one can think of no reasonable argument for them behaving differently. To me, both should examine a string to determine whether it is actually a number, obviously 1234 is a number, but is "1234"?