even if no pure sword-arts, aikido and hapkido use swords, so you logically can learn sword basics from them.
and of corse the basics are the same. sword gripping, thrusting, slashing, blocking they do equally and why shouldn't they?
No, you can't "logically learn sword basics from them". You could learn Aiki-ken from an Aikido school that teaches it, that's about it. As far as "Hapkido sword"… honestly, the less said the better. There was no sword that Choi brought back from Japan, all that's taught as "Hapkido sword" is made up, and desperately flawed. To think that that could teach anything even related to actual sword usage is to have no idea of how these weapons operate.
Chris and others have made the important points, but it might be a good idea to back up and explain at a more basic level where they are coming from.
Legitimate sword arts are those which were created by people who actually had experience with combative use of the sword. People who had been in real sword fights. Who had seen real sword fights. Who had taught others who used real swords in real fights.
Since real sword fights are mostly a thing of the past*, legitimate sword arts are primarily historical traditions. A modern practitioner who creates his own sword art based on how he imagines a sword should be used is pretty much LARPing. Imagination is not a substitute for empirical experience.
*(There are still some areas of the world where the machete is a commonly carried work tool and is sometimes used in fights. Theoretically a new legitimate sword art could be created by someone from one of those places who had survived a number of fights involving use of the machete.)
There are some commonalities which can be found in different surviving historical sword arts, but there are also a lot of differences, based on factors such as the type of sword being used, the type of weapon being faced, the type or clothing or armor being worn, the tactical doctrine being applied, the historical context, and so on. There isn't really a "generic" template of a sword art which you can learn basics from.
If you practice an art which has incorporated "sword training", but that training has no link back to the people who actually used a sword in combat, then your "sword training" is really so much baton twirling. It's a dangerous baton, to be sure. A sword is a long piece of sharpened steel and even an untrained person can easily hurt or kill others (or themselves) with a long piece of sharpened steel. An actual sword art goes beyond that and teaches you how to defeat someone else who has a deadly weapon and is trained in its use, while maximizing your chances for surviving the process. That's the sort of knowledge that someone had to bleed for.