Here’s Jimmy Akin’s issue as far as I understand it:
The authors of the letter don’t just accuse Pope Francis of maintaining a theologically heretical position. They accuse him of the specific crime of heresy. This is a canonical accusation and, as Akin points out, it requires an in-depth understanding of Canon Law in order to make such an accusation.
In terms of the theological positions that the authors point to as “proof” of the pope’s heresy, Akin points out that many sound theologians (including both the pope and the former head of the CDF) have explicitly said that those points and and must be interpreted in light of tradition and in a way that is consistent with tradition. The fact that the pope’s more ambiguous statements can be interpreted in a non-heretical way leaves no ground for accusing him of heresy. The fact that some of the more liberal-minded theologians interpret the pope’s teachings in a heterodox/heretical manner means that if anyone ought to be accused of heresy it’s those theologians, not the pope.
The other issues that Akin points out is that after the accusation of the crime of heresy, the authors point to the list of bishops and priests promoted by Pope Francis as “proof” of his heresy. But the fact that someone may not like the specific priest or bishop being promoted - even if that priest or bishop teaches heterodoxy - isn’t proof of a pope’s heresy.
Finally the authors attack some of the pope’s (more unfortunate) choices in… clothing. Ok. So perhaps it’s not quite that simple, but it’s basically what it boils down to. They claim that a crosier the pope used was a satanic staff (a heavy accusation that would require a great deal of proof and, as Akin points out, is very easily disproved) and that one of his pectoral crosses was a gay symbol (again, something that is quickly and easily disproved). To make an accusation of the canonical crime of heresy and then point to items of clothing one doesn’t like as “proof” of the heresy - while at the same time demonstrating that you don’t actually know the symbolism behind those articles of clothing (no matter how ugly the clothing actually is) - is irresponsible at best…