The problem with warranties and guarantees is in the fine print. Most people believe that if something is warranted for 25 years, it means that no matter what happens in that time period, they will get the project repaired or replaced.
However, once you start reading the fine print, there will be exclusions as to "normal wear and tear", which could include restrictions such as pets, children, heavy duty usage, etc. that will void the warranty. Or it could state that it is warranted to "resist fading" under "standard testing conditions", however, those testing conditions could have been partial shade versus full sun exposure; or any extreme weather such as temperatures below 0C (32F) which pretty much makes it worthless in Canada or northern US states.
Also, if it is a product warranty, then at best all that will be replaced is the cans of deck stain, and you would have to read the warranty to find out if it would be at current prices or if you have to produce the original receipt (if you held on to it all these years) and then you will only be reimbursed the cost of the stain from several years ago, and not inflated current prices.
Then, one still needs to pay the separate labour cost to remove the old product and re-stain the deck which is likely not part of the product warranty. I don't fully understand the deck staining business, so please correct me if I am wrong, but I assume that the greater cost is in the labour and not the materials.
So, maybe the warranty is a good selling ploy. They claim it lasts 25 years to give them an edge on the original sale; then hope that the client doesn't have the receipt to submit a valid claim but if the HO does still have it; then the contractor gets to make double money on the labour end by having a 2nd future job lined up to effect the replacement.
At best, I suppose you could warn the potential client to have a closer look at the warranty which will likely read "Limited Warranty", and this will already be their first clue to read a bit more thoroughly.
Warranties are similar to insurance. There is no such thing as being "fully covered". There are always exclusions and conditions. Anyone who gives a blanket statement of "fully warranted" or "fully insured" is misleading their client.