"OU by technicality," in addition to being used for cases where a mega evolution is less popular than that same Pokemon without the mega stone, is also used as a historical monument of sorts. In this case, the gen5 formats aren't played very much any more, so attempting to actively re-tier them all the time according to current usage stats isn't very productive--the sample size is much smaller, so one player going on a binge and playing a lot of matches would have a much more outsized effect on the weighted representation than they would in a more popular format like the current generation.
The Pokemon in question were used enough to maintain the threshold for OU at the end of the time gen5 formats were the main feature, but changes in player preferences in the years since have made them altogether less popular. In Dugtrio's case, there's also the additional factor that the sole reason for its viability, the Arena Trap ability, was later banned from the tier, and the only abilities it's left with aren't enough to make it any good. Rather than demote these Pokemon to UU, and leave the barely-existent gen5uu player base to have to test whether it's healthy to let those Pokemon exist in the format, they pull out the technicality label. What it means in this case is "When last we cared about such things, this Pokemon was popular enough for the standard OU label, but it no longer is today. We're notifying you of this as a courtesy in case you're browsing through the teambuilder to look for viable Pokemon to add to your team in a format you may not be very familiar with--this Pokemon probably won't lead to as much success as the ones that are still OU by usage, though of course you're still allowed to use it if you want."
