Hi abbygrant,
Not surprisingly I would agree with their suggestion.
There is more information about support systems (bedframes and foundations or platform beds) that are generally suitable for different types of mattresses and some examples of each of them in post #1 here and some of the information and comments in this topic (which would apply to any foam mattress) may be helpful as well.
Any mattress with a polyfoam support core will generally do best with a firm, flat, and evenly supportive support surface underneath it that has minimal to no flex under the mattress and for larger sizes with at least one center support beam that has good support to the floor to prevent any sagging in the middle of the mattress. The components need to be strong and durable enough to support the weight of the mattress and the people sleeping on it without some of the parts bending, sagging, shifting, or breaking with extended use. The support surface under the mattress should have enough surface area to prevent the mattress from sagging through any gaps or spaces in the support surface over time but still allow some airflow under the mattress. I would suggest that the gaps between any slats are no more than about 5" (with 1 x 3 slats) although less than 4" would be better yet.
Phoenix