CPES is designed to penetrate the upper few layers of wood cells and then cure, thus sealing the wood cells from moisture intrusion. It is NOT an adhesive in the strictest sense of the word.
So while if injected into various places drilled into a rotted subdeck core it will cure and seal up the individual wood cells it penetrates, it will NOT glue the whole thing into an impenetrable mass unless you can get it to soak into every rotten and good wood cell in the bad area. Which you can't do by drilling a few holes into the bad section. You'll seal up the first few layers of wood cells surrounding each hole, but it won't penetrate into the cells out between the holes.
CPES is designed to do one thing, and it does that one thing very well. It is NOT a cure-all for large areas of rotted wood such as one gets in a subdeck core. CPES is a sealant, NOT a strengthener.
For example, in the CPES instructions that come from Rot Doctor here in Seattle, a re-seller of Smiths CPES, the illustrated cure for a rotted beam end is to FIRST cut out ALL the rotten wood. Every little bit. Then paint the remaining end of the beam with multiple coats of CPES.
THEN.... use another Rot Doctor product, a two-part epoxy paste not unlike Bondo (but better) to create a new beam end. When this cures up hard, shape it the form you want. Then paint it with several coats of CPES just to seal the surface of the cured and shaped epoxy beam end, and then paint it or whatever.
So note the VERY important step of removing ALL the rotted wood first.
CPES will seal up wood cells,but it doesn't add appreciably to the strength of the piece of wood itself. The rotten wood will still be rotten and weak if you leave it in place. It will just be somewhat sealed up. That in itself will add some strength, but nothing approaching the integrity of the original wood core.
CPES is a great tool for what it's designed to do but it's not a magic bullet by any means.
Fixing a rotted subdeck core is one of those jobs that is either done correctly or it might as well not be done at all. Be nice if there was a quick and easy fix, but there isn't.