"Note that titled players are allowed to have a secondary account and some players may have requested permission to use a secondary account for e.g. blindfold games or a speedrun."
Titled players may have a second account to protect their anonymity iirc. The second part of the sentence should be self-explanatory. Of course, in either case, if rules are broken, the accounts are restricted and/or banned.
Why are titled players allowed to have a alt. If you go to report and press "multiple accounts" it says "The player uses multiple accounts without permission from Chess.com. Note that titled players are allowed to have a secondary account and some players may have requested permission to use a secondary account for e.g. blindfold games or a speedrun."