Considering the value of Azov being alive to UA and RU sides, both claims sound ridiculous to the point of being laughable. So most likely this was an accidental attack. As these things go, like with MH17, one side will try to do everything to investigate and drum up as much attention as possible, and the other side has no option but to just blatantly deny everything and try to make it go away ASAP.
It's even more ridiculous to say it was accidental attack.
The captured Azovites are still highly valued for exchange, because at large in Ukraine they are apparently still considered the elite and there are many more of their fighters and supporters at large who are pressing the Zelensky government to exchange them in the very first place.
For the Zelensky government, for which victories on the information front are much more important than real ones, it would be better if the Azov people did not surrender, but died the death of the brave as martyrs. Therefore, on the one hand, Kyiv exchanges part of the soldiers, and on the other hand, instead of Russia, it may well seek to complete its plan to glorify the prisoners from Azovstal by completely destroying it. Moreover, as mentioned above, such attacks may be designed not only to intimidate witnesses, but to reduce the desire of Ukrainian soldiers to surrender into Russian captivity.