Creating a new account and playing at a consistent level is not against the rules. Creating a new account, then throwing matches so you can steamroll later is against the rules and 343 has systems in place to ban those players
If the competitive skill system works in Halo as it does for Overwatch, then new accounts will almost always start on the lower elo spectrum. If the player blatantly holds back despite being skilled, then the system will keep them lower (or as you say, if they blatantly throw, they won't excel, they're cheating a system) and they can easily stomp the competition and then repeat the cycle the next season during placements.
I however don't know EXACTLY how the MMR works in Halo 5, and I won't pretend to know how it works; I am merely stating that if it works like another game, that's likely what's happening.
Regardless, if someone of high skill jumps onto an account that purposely is set to a lower elo to stomp the competition, that's hardly fair. I mean (again, comparing to Overwatch as that's the only other game I know of personally that has this problem) if a top500 player that plays, say, Reaper were to hop on an account they purposely kept at around Silver or Gold to easily stomp the competition; that's about a 2000 SR difference (which is about a 5 rank difference as Overwatch doesn't have intermission ranks, they just go Bronze, Silver, Gold, Platinum, Diamond, Master, Grand Master, top500). Things like that should not be allowed, yet they are, until they break something like a rule or someone finally decides they've seen enough reports to finally do something (or however it goes down.)
Doing things like that skews data; if it's in a team based scenario then some teams might experience victories they should not have and will push them up when they're not ready.
