The "if" is key. The parent post does not indicate that their voting residence is in Massachusetts - they could have maintained a voting residence in Pennsylvania instead of switching to MA.

Voting residency requirements are weird because they differ from state to state, and can be very vague when it comes to maintaining residency. As far as I can tell, Pennsylvania's 30-day residence requirement only applies when registering to vote; as long as you maintain a voting residence in Pennsylvania, you can vote in subsequent elections as you do not need to register again between elections. According to Article VII of Pennsylvania's election code (qualifications of electors), however, you lose your residence if you move to another state AND do not intend to return to Pennsylvania OR you move to another state AND intend to make it your permanent residence. So it depends on the parent poster's intentions of whether they view MA as their permanent residence or they don't plan to return to PA.

The post is worded like they were in college at the time. College students can register to vote in either their home state or the state they attend school in, just not both.

There is nothing on their post that implies age or student status, unless you think staying with your parents is something that only college students do and is not possible after graduation.

There also isn't anything in their post to say that they're not. But you're assuming it was illegal, based on... what? Why assume the worst rather than the more charitable assumption?

and you're assuming it was legal, based on... what? Why assume the best rather than the more realistic assumption?

Right, so you’re taking the least generous interpretation of the OPs statements and concluding they broke the law.

Please don’t that, it’s bad faith and needlessly divisive.

Are you a student? Most states don't automatically consider you a resident in that case. You won't change your driver's license, and you probably won't pay state taxes to that state.

Are you a contractor, and for how long? Again, you probably don't automatically change residency until you pass some magic number of days.

Many of the state laws are often a bit vague about the corner cases like students or contractors. The general principle seems to be "As long as you only vote once, in one place, you probably are within the bounds of most laws.

As always, YMMV, IANAL, etc.