Kuwait340, to answer your question,…no, it is not written/stated anywhere that you must level off when descending to decelerate in any ATC manual that I am aware of.
Quite possibly you will find this sort of procedure in some parameter operating manual for various aircraft, but definitely not any ATC manual.
It sounds to me like this chap simply threw validations out there to sew his reasoning together as you placed very valid questions to him.
If an ATC wants you to level off at any FL or altitude, he/she will clearly state so. If they don’t and upon questioning there-of by the pilot re-affirm that that is what they expected you to do without instructing it………….well,….hate to say it, but that pretty much starts falling into the much taboo category of controlling that all ATC’s are warned against and that is “assumption”. Most ATC’s in their training, either at recognized training institutions or post Course training will definitely be told that by one of their in-field OJTI’s.
The reason is quite simple; assumption is the Mother of all F**k Ups!
There is an old tried and tested adage from the vestiges of training methodologies in ATC circles (and other corporate arena’s) that still holds very true today. It is called the K.I.S.S. principle. Keep It Safe & Simple.
Most good ATC’s stick almost religiously to this principle as it virtually (not completely) eliminates any form of assumption or misinterpretation/understanding.
As you correctly stated you can bleed that speed off while descending, no need to level off. The ambiguity window here is rather wide as I see it, because if this chap did as he says expect you to level of, what are the parameters in miles/distance that his current traffic pattern, sequencing/ATM can allow you to level off at any altitude before it starts to become a problem in that it can adversely affect his current plan??........and surely if no parameter is stated then this will allow every pilot he gives this instruction to depending on various elements, interpret this in their own way?
This instruction is normally given in the following manner, “ (Acft Reg)…descend 3000’, passing 10000’, reduce 250kts, reaching 3000’ reduce 220kts..”
It is a four part instruction and some pilots might even ask for it to be repeated, but there are no area’s open for assumption.
My experience has been that if a pilot deems it necessary to level off to reduce speed (does not happen that often), he will always advise me of this.