Not sure if there is some confusion with the terms for the Portuguese CAA, maybe that's why they telling you that.
If you renew your ME/IR, but dont have valid MEP, you will not have a valid aircraft that you can use your rating on, except SEP.
Maybe this is what they refer to, the actual ME/IR does not expiry, when your MEP expires, but you will not be legal to fly PIC on any MEP with your MEIR rating.
If they say otherwise, as you have explained, than they are wrong. Why don't you check with the school where you plan to renew your ME/IR directly, as if in same country as issuer of your licence, they will sign your licence renewal, and they will be able to confirm to you that it is like this.
Note that if you are taking a renewal in another country, like Portuguese licence, and renewing in UK, this is possible, but you need to check what Portuguese CAA requires of documents from you and from the FTO.
I recall when I did, they needed the schools JAA authorization, the sims confirmation of standard, the instructor JAA examiner authorization, his licence, his medical. The UK FTO was not allowed to sign my licence, I had to send it to my CAA, and they reissued be a new licence. (MEIR valid) MEP expired.