We need to be specific here; Acorn BBC BASIC also lets you use inline assembly, and by all accounts, seems to be more powerful than most Forth assemblers I've seen.
Actually, no we don't have to be specific. BBC BASIC seems to be an outlier of the large array of BASIC implementations, especially BASICs on micros. In contrast, the statements about Forth (especially micro Forths) are widely supported. Yet there are certainly examples of Forth that did not have such a facility.
So saying "Forth has one more advantage over BASIC, the use of low level code" as a broad, general statement is perfectly valid.
And the assembler in BBC basic doesn't address the high level extensibility of Forth as particularly manifest by the existent of Forth assemblers. FIG-Forth, as an example, does NOT come with an assembler. It starts as an assembly source file, that compiled in to an operating Forth system. But it's quite possible, without access to the original FIG assembly source listing, to create a first class assembler in Forth, in to something that one would be hard pressed to discern was not "built in" to the system.
This first class extensibility of Forth is vastly different from BASIC in the large, and BBC BASIC specifically. However good the assembler in BBC BASIC is, that's as good as it's going to get. It can't be readily augmented, it can't be readily replaced, it can't be anything. Not by the end user, at least. You get what you get since you can not recreate BBC BASIC readily yourself. The extensibility of BASICs in general is quite limited in comparison to Forth.
