Yes today it would be welded together from smaller pieces but how do you forge something that size?
At that point in time as far as we know mechanical forging
machines were not in existence.
Yet huge forgings were made, such as the Pillar of Delhi
although that was somewhat closer to the present but
still pre-machine age.
It is vastly larger than that anchor and of much finer
workmanship. It also is known for not rusting or oxidizing
even exposed to the outdoors for hundreds of years.