In 1780, the Revolution was still going on. Tories were losing their property and being driven into exile. In that context a court decision abolishing slavery doesn’t seem like a terrible thing. There had already been such a decision in Britain, so there was precedent, and the population, inspired by ideas of freedom, did not reject or rebel against the decision. People who might like the courts to declare government intrusions and usurpation unconstitutional might not be quick to condemn the Massachusetts decision. Just what is and what isn’t illegitimate judicial activism isn’t always clear.
The challenge also is that this is not including the Massachusetts State constitution. (The version that was in place at the time) The states have different rules for their judicial branches.
This was not a USSC case, it was completely contained in the state so the US Constitution does not apply.
This is a good example where the state constitutions don’t get the respect they ought to, when in reality the state constitutions are just as important and even sometimes more important than the US Constitution.