I'd see nothing wrong with that. They say "usually", so it isn't certain.
Just say you'd like to know if your manuscript is under consideration, so you can solicit other publishers if it is not.
If it is worth their time, they will answer you. If it isn't they will probably tell you so.
Give them a few weeks to respond, and proceed with soliciting other publishers. If anybody comes back with a deal, then you can cancel the process with other publishers.
I don't know the process of textbook publishing, but certainly when looking to publish fiction, you approach a handful of agents or publishers at a time, and take the first deal that comes.
Only a handful, not dozens or hundreds. The reason for that is if you get rejected by the handful and they are kind enough to provide any criticism, you can address their criticisms and try again. Not with them, but with fresh agents. You don't want to approach everybody, get rejected by everybody, and the few that offer any criticism are united and it is something you can fix.
I'd put your own time limit on how long they have to respond before you go ahead and solicit another publisher. They say a month, you've given them two, I personally would just move on and submit to the next.
If the first finally expresses interest, stay engaged, tell them when you didn't hear from them you reached out to others, but you would like to work toward a commitment. Be honest but don't make promises of exclusivity and don't burn any bridges. Your goal is to get published, you don't owe anybody loyalty until you have their signature on a contract.
For example, if your first choice comes back with interest, and you cancel with your second choice, and then your first choice decides against making an offer -- What do you tell your second choice? Please take me back? I didn't mean it?
It's like dating. You're on the market. You're talking to lots of guys. Don't take yourself off the market until they put a ring on it.