Your example is a poor one, since no reasonable person could confuse those two items.
A reasonable person could confuse the plans produced by the website and legitimate plans produced by a professional surveyor, though.
More to the point, the author of the website seems to disagree with you, since he already includes a disclaimer on the website. The issue raised is that he does not include that same disclaimer on the plans.
GP's phrasing of 'everyone who will ever handle' the plans is the correct phrasing: those are the people that must be considered. That group includes all people whose work and interests require knowing where property boundaries and physical structures are, now, and in the future.
The professional engineering and survey boards have a duty to the protect the public, and a duty to regulate the practice of land surveying and civil engineering. If someone's behavior interferes with that practice, then they have a legitimate cause to prevent that behavior.
If I work in an office that requires plans to be done by a professional surveyor, it's my job to authenticate submissions I receive. If that requirement doesn't exist, then it doesn't matter. Your argument seems to depend on wholly imaginary future victims.
The disclaimer on the website is so buyers know what they're getting and don't waste money on something that isn't adequate for their legal/planning needs.
Office workers approving applications are not the only people who will see these. Users may include (disgruntled) neighbours, and anyone who will at some point need to know what the measurements on a site are to do their job correctly, legally, and/or safely. The possibility for misinterpretation is clear, and the fix is trivial.
Your interpretation of the disclaimer may be correct, but the phrasing "This is not a Legal Survey, nor is it intended to be or replace a Legal Survey." is excessive for that purpose and makes more sense if it's meant to head off complaints from the engineer's association.
A reasonable person could confuse the plans produced by the website and legitimate plans produced by a professional surveyor, though.
More to the point, the author of the website seems to disagree with you, since he already includes a disclaimer on the website. The issue raised is that he does not include that same disclaimer on the plans.
GP's phrasing of 'everyone who will ever handle' the plans is the correct phrasing: those are the people that must be considered. That group includes all people whose work and interests require knowing where property boundaries and physical structures are, now, and in the future.
The professional engineering and survey boards have a duty to the protect the public, and a duty to regulate the practice of land surveying and civil engineering. If someone's behavior interferes with that practice, then they have a legitimate cause to prevent that behavior.