Matt Whiting said something like:

....[snip]...

I didn't see a column length in the OP, but when "basement" is the
description rather than "crawl space" it is likely that the length is
at least 6' and possibly 8' or even more. This is far from being a 1'
column. Concrete filled PVC could work if sized properly, but using
the same size as the existing steel column is a fool's errand. Again, the
OP didn't mention the size of either the steel or the PVC, but I'll
bet there were likely nearly the same size. If that is the case,
then the PVC is almost certainly inadequate, unless the steel column
was grossly oversized for the required load.

In my original post I believe I was clear in using the steel column
reference only in what the person I met had used as his reasoning for
filling them with concrete.

I pointed out that HE was under the impression that the strength of the
steel column was primarily the concrete within it, and he believed that the
steel was a mere form for it. Hence he figured that using nearly anything
as a form, e.g. PVC, would be enough for the deck supports.

I am not sure of the diameter. I am not sure of the length. I am not sure
that they were also the footings (though I hope not, since I doubt he
understands the notion of "big feet" and I doubt he was using hugely wide
posts).

But I have seen concrete cracks in all kinds of things, and I can't help but
imagine a crack going clear through one of those things causing a disaster.
YMMV {shrug}