Gain figures are a best case. They assume perfect ground and no
losses of any kind. Real ground costs 1–2 dB for a wire antenna and
a lot more for a ground-mounted vertical or a short whip. Where that matters,
the page says so above.
It does not say whether the frequency will work. It says whether
your antenna points where the path needs it. Whether the ionosphere will bend
that frequency back down to your target is a separate question — use the
path planner or
reach
map. A green answer here means the antenna is not your problem, not that
the contact will happen.
It is not NEC. The pattern comes from the classical thin-wire model
over perfect ground. That is reliable for the shape of the pattern and the
angles it favours — it matches the textbook to a tenth of a dB where the
textbook has an answer — and it says nothing about feedpoint impedance,
SWR or bandwidth, so neither does this page.
Cut long and trim. Wire stretches, insulators add length, and
anything nearby shifts the tuning. Trust an antenna analyser over any model,
including this one. Where an issued antenna exists, its manual governs.