Yes it's a BBXB but it's also a Crossback Golden Arowana as all Blue Bases are essentially Golden Arowana. The cert may give an indication, but different farms use their somewhat different names for their fish, so it can be a little confusing: my BBXB (from one of the oldest farms in Malaysia) cert says "Malaysian Crossback Golden Arowana" -- because that's what is (even though it's clearly a Blue Base variation). Another farm may or may not mention "Blue Base" on the cert.
OP as to whether you should purchase an 18" Arowana, basically up to you, and if you have a bigger tank, because that fish still has a lot of growing to do. As others mention of course depends too on asking price.
On the plus side it's a nice looking BBXB and hopefully it should be more mellow than younger, 6"-12" Aros, which are more mentally fragile and tend to spook more easily.