Presumably if the speech therapist said she didn't see 'signs of autism', she was however, in agreement that there was some speech & language delays and offer help?
Not really sure if the SALT is equipped to see the 'signs', as it were but I may be wrong and if your instinct is that there is more, then you can request a referral to a Developmental Paediatrician. They may say it's too young to say - as they did with my son, where the SALT said possibly Dyxpraxia, however at 4.5, he has now been diagnosed ASD.
My eldest, now 6 was diagnosed earlier and my youngest didn't really display the same issues and was very different to his brother. Trust your gut instinct and put a referral request in now, as it may take some time to get an appointment.