Some developers have referred to this as a hack, is it accurate?
It was originally thought that segregated witness could only be added to Bitcoin using a hard fork. For that reason, it was considered unlikely to be possible to add to Bitcoin in the near term because hard forks are much harder to gain ecosystem-wide consensus for.
However later on, it was discovered that segregated witness could be added in backwards compatible manner as a soft fork. This method may have been described by segregated witness’s designers as a “hack” but this is more a colloquial term among programmers.
The actual difference is simply between the location of the witness commitment (in the block header for the hard fork version or in the coinbase transaction for the soft fork version). If one was designing Bitcoin from scratch, one would chose the block header, but this difference is not a “technical debt”.
In fact, the net effect of segregated witness actually cleans up a lot of technical debt rather than adding to it. If there ever a hard fork in the future, the location of the witness commitment could be relocated as was mentioned in the original roadmap proposal.