I'll say it again, FTTP is 100% unrelated to the PCP.
The PCP holds copper cables, no fibre at all.
The extension is to add extra space for more blocks for terminating copper cables, usually the tie pairs that link the PCP to the fibre cabinet.
Recently I've seen an extension being added just before a G.Fast cabinet.
So it's either for a G.Fast pod, a 2nd FTTC cabinet, or your cabinet is just absolutely stuffed and there's no space for more tie cables for the existing FTTC cabinet.
When the engineer says "it's for fibre" he likely means FTTC/G.Fast. It's all just referred to as "fibre".
The fibre used in FTTP has nothing to do with the PCP though. It comes from Aggregation nodes/Splitter nodes which are housed in underground chambers.