"The Road Not Taken" (1985) by Harry Turtledove
Confirmed correct in a comment by the OP. The Wikipedia summary is very similar to the description here:
The story is told through limited third person point of view, with most of the story concerning a single Roxolani captain, Togram. During a routine journey of conquest, they happen upon Earth. The Roxolani anticipate a simple and rewarding campaign, as they can detect no use of gravity manipulation, the cornerstone of their civilization. Humanity is awed by the invaders, as the maneuverability granted by that technology suggests the rest of their civilization is equally impressive. But as they begin their assault, things take a turn for the absurd—the Roxolani attack with matchlock weapons and black powder explosives. Humans retaliate with automatic weapons and missiles. The battle is short, and most of the invaders are killed. A few are captured alive.
When they are interrogated, the truth becomes evident: the method of manipulating gravity is absurdly simple, and species like the Roxolani are thus able to use faster than light travel with relatively primitive technological sophistication. This enabled them to engage in wars of conquest on a galactic scale. However, adopting the technology allowing for interstellar travel (and wars of conquest on a galactic scale) stifles further technological development. In contrast, humanity somehow missed developing gravity technology and advanced further technologically.
As Togram and another Roxolani captive realize the impact of the narrow but critical Roxolani technology for another, generally more advanced society, the story closes with the two asking themselves, "What have we done?"
Wikipedia, The Road Not Taken (short story)
"the story is quite short as far as I remember, maybe 3-5 pages"
One source I found online puts it at around 20 pages but this was a pdf copy of the original work.
"aliens look bear-like"
"Teddy bears!" Sandy Amoros exclaimed. The same thought had leaped into Cox's mind. The beings emerging from the spaceship were round, brown, and furry, with long noses and big ears. Teddy bears, however, did not normally carry weapons. They also, Cox thought, did not commonly live in a place that smelled like sewage. Of course it might have been perfume to them. But if it was, they and Earthpeople were going to have trouble getting along.
"The Road Not Taken"
"their weapons are very close to muskets"
"Slowmatches ready!" Togram said. The Roxolani lowered the smoldering cords to the toucholes of their muskets. "Take your aim!" The guns moved, very slightly. "Fire!"
"The Road Not Taken"
"as alien ships have only primitive sewerage facilities, the story points a couple of times to how much they stink."
The westerly breeze picked up. Cox's nose twitched. He could not name all the exotic odors wafting his way, but he recognized sewage and garbage when he smelled them. "God, what a stink!" he said.
"By the gods, what a stink!" Togram exclaimed. When the outer airlock doors went down, he had expected real fresh air to replace the stale, overused gases inside the Indomitable. This stuff smelled like smoky peat fires, or lamps whose wicks hadn't quite been extinguished. And it stung! He felt the nictitating membranes flick across his eyes to protect them.
"The Road Not Taken"
"at the end of the story, two captured aliens are shocked by the knowledge how much powerful will Earthlings become with the hyperdrive technology they have now (because with exception of hyperdrive Earthlings seem to them the most advanced tech civilization compared to other civilizations they know)"
"Well, maybe. But it's not just the weapons they have. It's the machines that let them see and talk to one another from far away; the machines that do their reckoning for them; their recorders and everything that has to do with them. From what they say of their medicine, I'm almost tempted to believe you and think they are wizards -- they actually know what causes their diseases, and how to cure or even prevent them. And their farming: this planet is far more crowded than any I've seen or heard of, but it grows enough for all these humans."
Togram sadly waggled his ears. "It seems so unfair. All that they got, just by not stumbling onto the hyperdrive."
"They have it now," Ransisc reminded him. "Thanks to us."
The Roxolani looked at each other, appalled. They spoke together: "What have we done?"
"The Road Not Taken"
