Yep. Prudent bet-hedging, I suppose.
Now there are some who will insist, with extreme uncharity to those who disagree with them, that Limbo definitely does not exist because it’s not in the Catechism and that those who think the salvation of unbaptized infants is unlikely just want babies to go to Hell, etc. Setting aside for a moment that the Catechism isn’t and isn’t meant to be an exhaustive summary of theological knowledge, even if it doesn’t teach it explicitly, there is much to recommend the idea of Limbo.
Limbo is in Hell, where Hell is understood to be where God is not seen and possessed. i.e., it is the absence of the beatific vision. One cannot have the beatific vision without being forgiven original sin, and (as far as we know) original sin cannot be forgiven without baptism. So, the idea goes, unbaptized babies are denied possession of God. But being innocent of actual sin, they do not suffer in Hell as do mortal sinners. Rather they possess every natural happiness, since God still wills what is good for them and they have not merited punishment. They have a better life than anything we could hope for on earth.
The idea that this is a difficult teaching is a strange one. God doesn’t owe us Heaven. It’s a supernatural good, i.e., one which transcends our natures, not a natural good, i.e., one which fulfills our natures. It is given to us as a gift, with strings attached; to think you’re entitled to a gift is, well, self-entitlement. That was the whole point of the Fall: Adam and Eve didn’t keep up their end of the bargain so the gift was taken from them. If anyone can be saved and baptism is irrelevant, then there was never a Fall. So as far as I can tell, belief in Limbo flirts much less with heresy than does belief that unbaptized babies are always and everywhere saved.
The arguments against Limbo, i.e., in the 2002 international theological commission document on the topic, seem pretty weak. They amount to “God wants everyone to be saved” and “It would be pastorally appropriate to speak as if all unbaptized babies are saved.” They don’t cut the mustard with the evidence we have to the contrary, i.e., the plain words of Christ re: the necessity of baptism and faith. The only point against Limbo I can think of is the fact that the Church has repeatedly considered teaching it de fide and has declined to. That’s a point in itself. Perhaps the Holy Spirit is protecting them from error. We probably can’t know if that’s the case this side of the grave.