This is a very exaggerated theory. Drugs spends a little to a while in your systems depends on where they have detected it. Following data from:
https://www.drugs.ie/drugs_info/about_drugs/how_long_do_drugs_stay_in_your_system/
Alcohol: 3-5 days in urine, 10-12 hours in blood
Amphetamines: 1-3 days in urine and around 12 hours in blood
Barbiturates: 2-4 days in urine and 1-2 days in blood
Benzodiazepines: 3-6 weeks in urine and 2-3 days in blood
Cannabis: 7-30 days in urine and up to 2 weeks in blood
Cocaine: 3-4 days in urine and 1-2 days in blood
Codeine: 1 day in urine and up to 12 hours in blood
Heroin: 3-4 days in urine and up to 12 hours in blood
LSD: 1-3 days in urine and up to 2-3 hours in blood
MDMA (ecstasy): 3-4 days in urine and 1-2 days in blood
Methamphetamine (crystal meth): 3-6 days in urine and 24 - 72 hours in blood
Methadone: 3-4 days in urine and 24-36 hours in blood
Morphine: 2-3 days in urine and 6-8 hours in blood
It may be true that Liam have consumed drugs prior to his death, but nothing is proven that his cause of death was from the consumption of drugs. Drugs may have affected his body/mind prior, but no sources can say that was his cause of death. For example, this is as exaggerated as saying that an individual died from a lollipop because they found it within their body.