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For an omnidirectional, I would tend to agree, generally speaking. There are only so many "tricks" you can do to make up for a lack of redirecting/focusing energy. Keep in mind that dBi is originally focused around directional antennas. Seeing 100+ dBi gain in a directional antenna is not uncommon. Especially in low power services such as cellular and Ham radio 1 watt global comms.

Some manufacturers use the theoretical isotropic antenna as a baseline and then compare their antenna to the isotropic. If they are honest they will specify their antenna gain in dBi. A dipole antenna has a gain of 2.15 dB above the isotropic and some manufacturers use the dipole as the baseline and specify their antenna gain in dBd. You can subtract 2.15 dB from dBi gain to match up with the dBd gain. Another consideration with gain is that it depends upon the angle of radiation. Some antennas may radiate with rather high angles, such as using a quarter wave vertical on the third harmonic which can give you a fairly close match. A good example being a 2 meter ham antenna being also used as a 440 MHz antenna. Even though the match might be reasonable, this type of antenna will have a higher angle of radiation on the odd harmonic and may not be as useful for VHF and higher frequencies, especially on flat land. The manufacturer could claim this level of gain without being specific as to the angle and makes the antenna appear to have higher gain than you might expect. There are cases where a higher angle might work better, such as if you were in a deep valley and wanted to communicate with a station at a higher elevation. But, most of the time we want the antenna to radiate toward the horizon for maximum communications distance.